
Book^JL3_ 



THE 

POWER OF MOVEMENT 
IN PLANTS 



BY 

CHARLES DARWIN, LL. D., F. R. S. 

ASSISTED BY 

FRANCIS DARWIN 



WITH ILLUSTRATIONS 



NEW YORK 
D. APPLETON AND COMPANY 

1900 



txi;^; 



Authorized Edition. 



7/ 



'"-c,^; 

A-^^ 



COIS'TEJS^TS 



Introduction Pages 1-8 

CHAPTER I. 

The Circumnutating Movements of Seedling Plants. 

Brassica oleracea, circum nutation of the radicle, of the arched hypo- 
cotyl whilst still buried beneath the ground, whilst rising above 
the ground and straightening itself, and when erect — Circumnuta- 
tion of the cotyledons — Eate of movement — Analogous observa- 
tions on various organs in species of Githago, Gossypium, Oxalis, 
Tropseolum, Citrus, ^sculus, of several Leguminous and Cucurbi- 
taceous genera, Opuntia, Helianthus, Primula, Cyclamen, Stapelia, 
Cerinthe, Nolana, Solanum, Beta, Eicinus, Quercus, Corylus, Pinus, 
Cycas, Canna, Allium, Asparagus, Phalaris, Zea, Avena, Nephro- 
dium, and Selaginella 9-68 

CHAPTER II. 

General Considerations of th;e Movements and Growth of 
Seedling Plants. 

Generality of the circumnutating movement — Eadicles, their circum- 
nutation of service— Manner in which they penetrate the ground 
— Manner in which hypocotyls and other organs break through 
the ground by being arched — Singular manner of germination in 
Megarrhiza, &c. — Abortion of cotyledons — Circumnutation of hypo- 
cotyls and epicotyls whilst still buried and arched — Tlieir power 
of straightening themselves — Bursting of the seed-coats — Inherited 
effect of the arching process in hypogean hypocotyls— Cii'cumnu- 
tatiou of hypocotyls and epicotyls when erect — Circumnutation of 
cotyledons— Pulvini or joints of cotyledons, duration of their activ- 

V 



VI CONTENTS. 

ity, rudimentary in Oxalis corniculata, their development — Sen- 
sitiveness of cotyledons to light and consequent disturbance of 
their periodic movements — Sensitiveness of cotyledons to contact. 

Pages 69-129 

CHAPTEfl III. 

Sensitiveness of the Apex of the Radicle to Contact and 
TO other Irritants. 

Manner in which radicles bend when they encounter an obstacle in 
the soil — Vicia faba, tips of radicles highly sensitive to contact 
and other irritants — Effects of too high a temperature — Power of 
discriminating between objects attached on opposite sides — Tips of 
secondary radicles sensitive — Pisum, tips of radicles sensitive — 
Effects of such sensitiveness in overcoming geotropism— Secondary 
radicles — Phaseolus, tips of radicles hardly sensitive to contact, 
but highly sensitive to caustic and to the removal of a slice — Tro- 
pseolum — Gossypium — Cucurbita — Eaphanus — iEsculus,tip not sen- 
sitive to slight contact, highly sensitive to caustic — Quercus, tip 
highly sensitive to contact — Power of discrimination — Zea, tip 
highly sensitive, secondary radicles — Sensitiveness of radicles to 
moist air — Summary of chapter 130-199 



CHAPTER IV. 

The Circumnutating Movements of the several parts of 
Mature Plants. 

Circumnutation of stems : concluding remarks on — Circumnutation of 
stolons : aid thus afforded in winding amongst the stems of sur- 
rounding plants — Circumnutation of flower-stems — Circumnuta- 
tion of Dicotyledonous leaves — Singular oscillatory movement of 
leaves of Dionsea — Leaves of Cannabis siuk at night — Leaves of 
Gymnosperms — Of Monocotyledons — Cryptogams — Concluding re- 
marks on the circumnutation of leaves : generally rise in the 
evening and sink in the morning 200-264 



CHAPTER V. 

Modified Circumnutation : Climbing Plants ; Epinastic and 
Hyponastic Movements. 

Circumnutation modified through innate causes or through the action 
of external conditions — lunate causes — Climbing plants ; similarity 



CONTENTS. Vll 

of their movements with those of ordinary plants ; increased am- 
plitude ; occasional points of difference — Epinastic growth of 
young leaves — Hyponastic growth of the hypocotyls and epicotyls 
of seedlings — Hooked tips of climbing and other plants due 
to modified circumnutation — Ampelopsis tricuspidata — Smithia 
Pfundii — Straightening of the tip due to hyponasty — Epinastic 
growth and circumnutation of the flower-peduncles of Trifolium 
repens and Oxalis carnosa Pages 265-282 

CHAPTER VI. 

Modified Circumnutation : Sleep or Nyctitropic Movements, 
THEIR Use ; Sleep of Cotyledons. 

Preliminary sketch of the sleep or nyctitropic movements of leaves — 
Presence of pulvini — The lessening of radiation the final cause 
of nyctitropic movements — Manner of trying experiments on 
leaves of Oxalis, Arachis, Cassia, Melilotus, Lotus and Marsilea, 
and on the cotyledons of Mimosa — Concluding remarks on radia- 
tion from leaves — Small differences in the conditions make a great 
difference in the result — Description of the nyctitropic position 
and movements of the cotyledons of various plants — List of species 
— Concluding remarks — Independence of the nyctitropic move- 
ments of the leaves and cotyledons of the same species — Seasons 
for believing that the movements have been acquired for a special 
purpose 283-319 



CHAPTER Vn. 

Modified Circumnutation : Nyctitropic or Sleep Movements 
of Leaves. 

Conditions necessary for these movements — List of Genera and Fami- 
lies, which include sleeping plants — Description of the movements 
in the several Genera — Oxalis : leaflets folded at night — Averrhoa : 
rapid movements of the leaflets — Porlieria : leaflets close when 
plant kept very dry — Tropseolum ; leaves do not sleep unless well 
illuminated during day — Lupinus : various modes of sleeping — 
Melilotus : singular movements of terminal leaflet— Trifolium — 
Desmodium : rudimentary lateral leaflets, movements of, not de- 
veloped on young plants, state of their pulvini — Cassia : complex 
movements of the leaflets — Bauhinia : leaves folded at night — 
Mimosa pudica : compounded movements of leaves, effect of dark- 
ness — Mimosa albida, reduced leaflets of — Schrankia : downward 



Vlll CONTENTS. 

movement of the pinnae — Marsilea : the only cryptogam known to 
sleep— Concluding remarks and summary — Nyctitropism consists 
of modified circumnutation, regulated by the alternations of light 
and darkness — Shape of first true leaves . . Pages 320-422 



CHAPTER VIII. 

Modified Circumnutation : Movements excited by Light. 

Distinction between heliotropism and the effects of light on the perio- 
dicity of the movements of leaves — Heliotropic movements of Beta, 
Solanum, Zea, and Avena — Heliotropic movements towards an 
obscure light in Apios, Brassica, Phalaris, Tropseolum, and Cassia 
— Apheliotropic movements of tendrils of Bignonia — Of flower- 
peduncles of Cyclamen — Burying of the pods — Heliotropism and 
apheliotropism modified forms of circumnutation — Steps by which 
one movement is converted into the other — Transversal heliotrop- 
ismus or diaheliotropism influenced by epinasty, the weight of 
the part and apogeotropism — Apogeotropism overcome during the 
middle of the day by diaheliotropism — Effects of the weight of 
the blades of cotyledons — So-called diurnal sleep — Chlorophyll 
injured by intense light — Movements to avoid intense light. 

423-453 

CHAPTER IX. 

Sensitiveness of Plants to Light : its transmitted effects. 

Uses of heliotropism — Insectivorous and climbing plants not helio- 
tropic — Same organ heliotropic at one age and not at another — 
Extraordinary sensitiveness of some plants to light — The effects 
of light do not correspond with its intensity — Effects of previous 
illumination — Time required for the action of light — After-effects 
of light— Apogeotropism acts as soon as light fails— Accuracy with 
which plants bend to the light — This dependent on the illumina- 
tion of one whole side of the part — Localised sensitiveness to light 
and its transmitted effects— Cotyledons of Phalaris, manner of 
bending— Eesults of the exclusion of light from their tips — Effects 
transmitted beneath the surface of the ground— Lateral illuraiua- 
tion of the tip determines the direction of the curvature of the 
base — Cotyledons of Avena, curvature of basal part due to the 
illumination of upper part— Similar results with the hypocotyls of 
Brassica and Beta — Eadicles of Sinapis apheliotropic, due to the 
sensitiveness of their tips— Concluding remarks and summary of 



CONTENTS. IX 

chapter — Means by which circumnutation has been converted into 
heliotropism or apheliotropism .... Pages 454-496 

CHAPTER X. 

Modified Circumnutation : Movements excited by 
Gravitation. 

Means of observation — Apogeotropism — Cytisus — Verbena — Beta — 
Gradual conversion of the movement of circumnutation into apo- 
geotropism in Eubus, Lilium, Phalaris, A vena, and Brassica — 
Apogeotropism retarded by heliotropism — Effected by the aid of 
joints or pulvini — Movements of flower-peduncles of Oxalis — Gen- 
eral remarks on apogeotropism — Geotropism — Movements of radi- 
cles — Burying of seed-capsules — Use of process — Trifolium subter- 
raneum — Arachis — Amphicarpsea — Diageotropism — Conclusion. 

497-526 

CHAPTER XI. 

Localised Sensitiveness to Gravitation, and its transmitted 
effects. 

General considerations — Vicia faba, effects of amputating the tips of 
the radicles — Eegeneration of the tips — Effects of a short exposure 
of the tips to geotropic action and their subsequent amputation — 
Effects of amputating the tips obliquely — Effects of cauterising the 
tips — Effects of grease on the tips — Pisum sativum, tips of radicles 
cauterised transversely, and on their upper and lower sides — 
Phaseolus, cauterisation and grease on the tips — Gossypium — 
Cucurbita, tips cauterised transversely, and on their upper and 
lower sides — Zea, tips cauterised — Concluding remarks and sum- 
mary of chapter — Advantages of the sensibility to geotropism being 
localised in the tips of the radicles 527-550 



CHAPTER XII. 

Summary and Concluding Remarks. 

Nature of the circumnutating movement — History of a germinating 
seed — The radicle first protrudes and circumnutates — Its tip 
highly sensitive — Emergence of the hypocotyl or of the epicotyl 
from the ground under the form of an arch — Its circumnutation 
and that of the cotyledons — The seedling throws up a leaf-bearing 



X CONTENTS. 

Gtem — The circumnutation of all the parts or organs — Modified 
circumnutation — Epinasty and hyponasty — Movements of climb- 
ing plants — Nyctitropic movements — Movements excited by light 
and gravitation — Localised sensitiveness — Eesemblance between 
the movements of plants and animals — The tip of the radicle acts 
like a brain Pages 551-576 

Index 577-592 



THE MOVEMENTS OF PLANTS. 



INTEOBUCTION. 

The chief object of the present work is to describe 
and connect together several large classes of movement, 
common to almost all plants. The most widely preva- 
lent movement is essentially of the same nature as that 
of the stem of a climbing plant, which bends successively 
to all points of the compass, so that the tip revolves. 
This movement has been called by Sachs "revolving 
nutation ; " but we have found it much more conveni- 
ent to use the terms circumnutation and circumnutate. 
As we shall have to say much about this movement, it 
will be useful here briefly to describe its nature. If we 
observe a circumnutating stem, which happens at the 
time to be bent, we will say towards the north, it will be 
found gradually to bend more and more easterly, until it 
faces the east ; and so onwards to the south, then to the 
west, and back again to the north. If the movement 
had been quite regular, the apex would have described a 
circle, or rather, as the stem is always growing upwards, 
a circular spiral. But it generally describes irregular 
elliptical or oval figures ; for the apex, after pointing in 
any one direction, commonly moves back to the opposite 
side, not, however, returning along the same line. After- 
wards other irregular ellipses or ovals are successively 
described, with their longer axes directed to different 
points of the compass. Whilst describing such figures, 
the apex often travels in a zigzag line, or makes small 

1 



2 INTRODUCTION. 

subordinate loops or triangles. In the case of leaves the 
ellipses are generally narrow. ' 

Until recently the cause of all such bending move- 
ments was believed to be due to the increased growth of 
the side which becomes for a time convex ; that this side 
does temporarily grow more quickly than the concave 
side has been well established ; but De Vries has lately 
shown that such increased growth follows a previously 
increased state of turgescence on the convex side.* In 
the case of parts provided with a so-called joint, cushion 
or pulvinus, which consists of an aggregate of small cells 
that have ceased to increase in size from a very early age, 
we meet with similar movements ; and here, as Pfeifer 
has shown f and as we shall see in the course of this 
work, the increased turgescence of the cells on opposite 
sides is not followed by increased growth. Wiesner de- 
nies in certain cases the accuracy of De Vries' conclusion 
about turgescence, and maintains J that the increased 
extensibility of the cell-walls is the more important ele- 
ment. That such extensibility must accompany increased 
turgescence in order that the part may bend is manifest, 
and this has been insisted on by several botanists ; but 
in the case of unicellular plants it can hardly fail to be 
the more important element. On the whole we may at 
present conclude that increased growth, first on one side 
and then on another, is a secondary effect, and that the 
increased turgescence of the cells, together wdth the ex- 
tensibility of their walls, is the primary cause of the 
movement of circumnutation.* 



* Sachs first showed ('Lchr- t ' Die Peri odischen Bewegun- 

buch,' &c., 4th edit. p. 452) tlie gen der Blattorgane,' 1875. 

intimate connection betweeii tnr- X ' Untersuchungen iiher den 

gescence and growth. For Do Heliotropismus,' Sitzb. der K. 

Vries' interesting essay, 'Wachs- Akad. der Wissenschaft. (Vienna), 

thnniskriinimungen niehrzclliger Jan. 1880. 

Organe,' see ' Bot. Zeitung,' Dec. * See Mr. Vines' excellent dis- 

19, 1879, p. 830. cussion ( ' Arbeiten des Bot. lusti- 



INTRODUCTION. 3 

In the course of the present Yolume it will be shown 
that apparently every growing part of every plant is con- 
tinually circumnutating, though often on a small scale. 
Even the stems of seedlings before they have broken 
through the ground, as well as their buried radicles, cir- 
cumnutate, as far as the pressure of the surrounding 
earth permits. In this universally present movement we 
have the basis or groundwork for the acquirement, ac- 
cording to the requirements of the plant, of the most 
diversified movements. Thus, the great sweeps made by 
the stems of twining plants, and by the tendrils of other 
climbers, result from a mere increase in the amplitude 
of the ordinary movement of circumnutation. The posi- 
tion which young leaves and other organs ultimately 
assume is acquired by the circumnutating movement be- 
ing increased in some one direction. The leaves of vari- 
ous plants are said to sleep at night, and it will be seen 
that their blades then assume a vertical position through 
modified circumnutation, in order to protect their upper 
surfaces from being chilled through radiation. The 
movements of various organs to the light, which are so 
general throughout the vegetable kingdom, and occasion- 
ally from the light, or transversely with respect to it, are 
all modified forms of circumnutation ; as again are the 
equally prevalent movements of stems, &c., towards the 
zenith, and of roots towards the centre of the earth. In 
accordance with these conclusions, a considerable diffi- 
culty in the way of evolution is in part removed, for it 
might have been asked, how did all their diversified 
movements for the most different purposes first arise? 
As the case stands, we know that there is always move- 



tuts in Wiirzburg,' B. IT. pp. 142, 1874, p. 211) on the curious move- 

143, 1878) on this intricate subject. ments of Spirogyra, a plant con- 

Hofmeister's observations ( ' Jah- sisting of a single row of cells, ai'e 

reschrifte des Vereins fiir Vaterl. valuable in relation to this sub- 

Naturkunde in Wiirtemberg,' ject. 



4 INTRODUCTION. 

ment in progress, and its amplitude, or direction, or 
both, have only to be modified for the good of the plant 
in relation with internal or external stimuli. 

Besides describing the several modified forms of cir- 
cumnutation, some other subjects will be discussed. The 
two which have interested us most are, firstly , the fact 
that with some seedling plants the uppermost part alone 
is_sensitive to light, and transmits an influence to the 
lower part, causing it to bend. If therefore the upper 
part be wholly protected from light, the lower part may 
be expp,sad for hours to it, and yet does not become in 
the least bent, although this would have occurred quickly 
if the upper part had been excited by light. Secondly, 
with the radicJes of seedlings, the tip is sensitive to vari- 
ous stimuli, especially to very slight pressure, and, when 
thus excited, transmits an influence to the upper part, 
causing it to bend from the pressed side. On the other 
hand, if the tip is subjected to the vapour of water pro- 
ceeding from one side, the upper part of the radicle 
bends towards this side. Again it is the tip, as stated 
by Ciesielski, though denied by others, which is sensitive 
to the attraction of gravity, and by transmission causes 
the adjoining parts of the radicle to bend towards the 
centre of the earth. These several cases of the effects of 
contact, other irritants, vapour, light, and the attraction 
of gravity being transmitted from the excited part for 
some little distance along the organ in question, have an 
important bearing on the theory of all such movements. 

Terminology. — A brief explanation of some terms which will 
be used, must here be given. With seedlings, the stem which 
supports the cotyledons (i.e. the organs which represent the 
first leaves) has been called by many botanists the hypocotyled- 
onous stem, but for brevity sake we will speak of it merely as 
the liyi^ocotyl : the stem immediately above the cotyledons will 
be called the epicotyl or plumule. The radicle can be distin- 



INTRODUCTION. 5 

guished from the hypocotyl only by the presence of root-hairs 
and the nature of its covering. The meaning of the word cir- 
cumnutation has already been explained. Authors speak of 
positive and negative heliotropism,* — that is, the bending of an 
organ to or from the light ; but it is much more convenient to 
confine the word heliotropism to bending towards the light, and 
to designate as apTieliotropism bending from the light. There 
is another reason for this change, for writers, as we have ob- 
served, occasionally drop the adjectives positive and negative, 
and thus introduce confusion into their discussions. Diaheli- 
otropism may express a position more or less transverse to the 
light and induced by it. In like manner positive geotropism, 
or bending towards the centre of the earth, will be called by us 
geotropism ; apogeotropism will mean bending in opposition to 
gravity or from the centre of the earth ; and diageotropism, a 
position more or less transverse to the radius of the earth. 
The words heliotropism and geotropism properly mean the act 
of moving in relation to the light or the earth ; but in the same 
manner as gravitation, though defined as "the act of tending 
to the centre," is often used to express the cause of a body fall- 
ing, so it will be found convenient occasionally to employ heliot- 
ropism and geotropism, &c., as the cause of the movements in 
question. 

The term epinasty is now often used in Germany, and im- 
plies that the upper surface of an organ grows more quickly 
than the lower surface, and thus causes it to bend downwards. 
Hyponasty is the reverse, and implies increased growth along 
the lower surface, causing the part to bend upwards.! 

Methods of Observation. — The movements, sometimes very 
small and sometimes considerable in extent, of the various 
organs observed by us, were traced in the manner which after 
many trials we found to be best, and which must be described. 
Pla nts growin g-inkpots were protected wholly from the light,, 
or had light admitted from above, or on one side as the case 



* The highly useful terms of f These terms are used in the 

Heliotropism and Geotropism sense given them by De Vries, 

were first used by Dr. A. B. Frank: ' Wiirzbnrg Arbeiten,' Heft ii. 

see his remarkable ' Beitrage zur 1872, p. 252. 
Pflanzenphysiologie, 1868. 



6 INTRODUCTION. 

might require, and were covered above by a large horizontal 
sheet of glass, and with another vertical sheet on one side. A 
glass filament, not thicker than a horsehair, and from a quarter 
to three-quarters of an inch in length, was affixed to the part 
to be observed by means of shellac dissolved in alcohol. The 
solution was allowed to evaporate, until it became so thick that 
it set hard in two or three seconds, and it never injured the 
tissues, even the tips of tender radicles, to which it was applied. 
To the end of the glass filament an excessively minute bead of 
black sealing-wax was cemented, below or behind which a bit 
of card with a black dot was fixed to a stick driven into the 
ground. The weight of the filament was so slight that even 
small leaves were not perceptibly pressed down. Another 
method of observation, when much magnification of the move- 
ment was not required, will presently be described. The bead 
and the dot on the card were viewed through the horizontal or 
vertical glass-plate (according to the position of the object), 
and when one exactly covered the other, a dot was made on 
the glass-plate with a sharply pointed stick dipped in thick 
Indian-ink. Other dots were made at short intervals of time 
and these were afterwards joined by straight lines. The figures 
thus traced were therefore angular; but if dots had been made 
every 1 or 2 minutes, the lines would have been more curvi- 
linear, as occurred when radicles were allowed to trace their 
own courses on smoked glass-plates. To make the dots accu- 
rately was the sole difficulty, and required some practice. Nor 
could this be done quite accurately, when the movement was 
much magnified, such as 30 times and upwards; yet even in 
this case the general course may be trusted. To test the accu- 
racy of the above method of observation, a filament was fixed to 
an inanimate object which was made to slide along a straight 
edge and dots were repeatedly made on a glass-plate; when 
these were joined, the result ought to have been a perfectly 
straight line, and the line was very nearly straight. It may be 
added that when the dot on the card was placed half-an-inch 
below or behind the bead of sealing-wax, and when the glass- 
plate (supposing it to have been properly curved) stood at a dis- 
tance of 7 inches in front (a common distance), then the tracing 
represented the movement of the bead magnified 15 times. 



INTRODUCTION. 7 

Whenever a great increase of the movement was not re- 
quired, another, and in some respects better, method of ob- 
servation was followed. This consisted in fixing two minute 
triangles of thin paper, about -^^ inch in height, to the two 
ends of the attached glass filament ; and when their tips were 
brought into a line so that they covered one another, dots were 
made as before on the glass-plate. If we suppose the glass- 
plate to stand at a distance of seven inches from the end of the 
shoot bearing the filament, the dots when joined, will give 
nearly the same figure as if a filament seven inches long, dipped 
in ink, had been fixed to the moving shoot, and had inscribed 
its own course on the plate. The movement is thus consider- 
ably magnified ; for instance, if a shoot one inch in length were 
bending, and the glass-plate stood at the distance of seven 
inches, the movement would be magnified eight times. It 
would, however, have been very difficult to have ascertained in 
each case how great a length of the shoot was bending ; and 
this is indispensable for ascertaining the degree to which the 
movement is magnified. 

After dots had been made on the glass-plates by either of 
the above methods, they were copied on tracing paper and 
joined by ruled lines, with arrows showing the direction of the 
movement. The nocturnal courses are represented by straight 
broken lines. The first dot is always made larger than the 
others, so as to catch the eye, as may be seen in the diagrams. 
The figures on the glass-plates were often drawn on too large 
a scale to be reproduced on the pages of this volume, and the 
proportion in which they have been reduced is always given.* 
Whenever it could be approximately told how much the move- 
ment had been magnified, this is stated. We have perhaps 
introduced a superflaous number of diagrams; but they take 
up less space than a full description of the movements. Almost 
all the sketches of plants asleep, &c., were carefully drawn for 
us by Mr. George Darwin. 

As shoots, leaves, &c., in circumnutating bend more and 
more, first in one direction and then in another, they were 
necessarily viewed at different times more or less obliquely; 



* We are much indebted to he has reduced and engraved our 
Mr. Cooper for the care with which diagrams. 



8 INTRODUCTION. 

and as the dots were made on a flat surface, the apparent 
amount of movement is exaggerated according to the degree 
of obliquity of the point of view. It would, therefore, have 
been a much better plan to have used hemispherical glasses, 
if we had possessed them of all sizes, and if the bending part 
of the shoot had been distinctly hinged and could have been 
placed so as to have formed one of the radii of the sphere. 
But even in this case it would have been necessary afterwards 
to have projected the figures on paper; so that complete accu- 
racy could not have been attained. From the distortion of our 
figures, owing to the above causes, they are of no use to any 
one who wishes to know the exact amount of movement, or the 
exact course pursued ; but they serve excellently for ascertain- 
ing whether or not the part moved at all, as well as the gen- 
eral character of the movement. 

In the following chapters, the movements of a con- 
siderable number of plants are described ; and the species 
have been arranged according to the system adopted by 
Hooker in Le Maout and Decaisne's ' Descriptive Bot- 
any.' No one who is not investigating the present sub- 
ject need read all the details, which, however, we have 
thought it advisable to give. To save the reader trouble, 
the conclusions and most of the more important parts 
have been printed in larger type than the other parts. 
lie may, if he thinks fit, read the last chapter first, as it 
includes a summary of the whole volume ; and he will 
thus see what points interest him, and on which he 
requires the full evidence. 

Finally, we must have the pleasure of returning our 
sincere thanks to Sir Joseph Hooker and to Mr. W. 
Thiselton Dyer for their great kindness, in not only 
sending us plants from Kew, but in procuring others 
from several sources when they were required for our 
observations ; also, for naming many species, and giving 
us information on various points. 



CHAPTER I. 

The Circumnutating Movements of Seedling Plants. 

Brassica oleracea, circumnutation of the radicle, of the arched hypo- 
cotyl whilst still buried beneath the ground, whilst rising above 
the ground and straightening itself, and when erect — Circumnuta- 
tion of the cotyledons — Eate of movement — Analogous observa- 
tions on various organs in species of Githago, Gossypium, Oxalis, 
Tropseolum, Citrus, ^sculus, of several Leguminous and Cucurbi- 
taceous genera, Opuntia, Helianthus, Primula, Cyclamen, Stapella, 
Cerinthe, Nolana, Solanutn, Beta, Eicinus, Quercus, Corylus, Pinus, 
Cycas, Canna, Allium, Asparagus, Phalaris, Zea, Avena, Nephro- 
dium, and Selaginella. 

The following chapter is devoted to the circumnu- 
tating movements of the radicles, hypocotyls, and cotyle- 
dons of seedling plants ; and, when the cotyledons do 
not rise above the gronnd, to the movements of the epi- 
cotyl. But in a future chapter we shall have to recur to 
the movements of certain cotyledons which sleep at 
night. 

Brassica oleracea {Cruciferoe). — Fuller details will be given 
with respect to the movements in this case than in any other, 
as space and time will thus ultimately be saved. 

Radicle. — A seed with the radicle projecting -05 inch was 
fastened with shellac to a Uttle plate of zinc, so that the radicle 
stood up vertically ; and a fine glass filament was then fixed 
near its base, that is, close to the seed-coats. The seed was 
surrounded by little bits of wet sponge, and the movement of 
the bead at the end of the filament was traced (Fig. 1) during 
sixty hours. In this time the radicle increased in length from 
•05 to •!! inch. Had the filament been attached at first close 
to the apex of the radicle, and if it could have remained there 
all the time, the movement exhibited would have been much 

9 



10 



CIRCUMNUTATION OF SEEDLINGS. Chap. I. 



N^ 



greater, for at the close of our observations the tip, instead of 

standing vertically upwards, had become bowed downwards 

through geotropism, so as 
^^" * - ^ almost to touch the zinc 

plate. As far as we could 
roughly ascertain by measure- 
ments made with compasses 
on other seeds, the tip alone, 
for a length of only ^|-g- to 
yfo" of an inch, is acted on 
by geotropism. But the trac- 
ing shows that the basal part 
of the radicle continued to 
circumnutate irregularly dur- 
ing the whole time. The 
actual extreme amount of 
movement of the bead at the 
end of the filament was nearly 
•05 inch, but to what extent 
the movement of the radicle 
was magnified by the fila. 

ment, which was nearly | inch in length, it was impossible to 

estimate. 

Another seed was treated and observed in the same manner, 

but the radicle in this case protruded '1 inch, and was not fast- 
Fig. 2. 




Brassica oleracea : circumnutation 
of radicle, traced on horizontal 
glass, from 9 a.m. Jan. 31st to 9 
P.M. Feb. 2nd. Movement of 
bead at end of filament magnified 
about 40 times. 




\ 



Brassica oleracea : circumnutating and geotropic movement of radicle, 
traced on horizonfcil glass during 46 hours. 

ened so as to project quite vertically upwards. The filament 
was affixed close to its base. The tracing (Fig. 3, reduced by 



Chap. I. BRASSICA. H 

half) shows the movement from 9 a.m. Jan. 31st to 7 a.m. Feb. 
3nd; but it continued to move during the whole of the 3nd in 
the same general direction, and in a similar zigzag manner. 
From the radicle not being quite perpendicular when the fila- 
ment was aflQxed geotropism came into play at once ; but the 
irregular zigzag course shows that there was growth (probably 
preceded by turgescence), sometimes on one and sometimes on 
another side. Occasionally the bead remained stationary for 
about an hour, and then probably growth occurred on the side 
opposite to that which caused the geotropic curvature. In the 
case previously described the basal part of the very short radicle 
from being turned vertically upwards, was at first very little 
affected by geotropism. Filaments were aflSxed in two other 
instances to rather longer radicles protruding obliquely from 
seeds which had been turned upside down ; and in these cases 
the lines traced on the horizontal glasses were only slightly zig- 
zag, and the movement was always in the same general direc- 
tion, through the action of geotropism. All these observations 
are liable to several causes of error, but we believe, from what 
will hereafter be shown with respect to the movements of the 
radicles of other plants, that they may be largely trusted. 

Hypocotyl. — The hypocotyl protrudes through the seed -coats 
as a rectangular projection, which grows rapidly into an arch 
like the letter U turned upside down n ; the cotyledons being 
still enclosed within the seed. In whatever position the seed 
may be embedded in the earth or otherwise fixed, both legs of 
the arch bend upwards through apogeotropism, and thus rise 
vertically above the ground. As soon as this has taken place, 
or even earlier, the inner or concave surface of the arch grows 
more quickly than the upper or convex surface ; and this tends 
to separate the two legs and aids in drawing the cotyledons out 
of the buried seed-coats. By the growth of the whole arch the 
cotyledons are ultimately dragged from beneath the ground, 
even from a considerable depth ; and now the hypocotyl quickly 
straightens itself by the increased growth of the concave side. 

Even whilst the arched or doubled hypocotyl is still be- 
neath the ground, it circumnutates as much as the pressure of 
the surrounding soil will permit; but this was difficult to ob- 
serve, because as soon as the arch is freed from lateral pressure 



12 CIRCUMNUTATION OF SEEDLINGS. Chap. I. 

the two legs begin to separate, even at a very early age, before 
the arch would naturally have reached the surface. Seeds were 
allowed to germinate on the surface of damp earth, and after 
they had fixed themselves by their radicles, and after the, as 

Fig. 3. 



Brassica oleracea : circum nutating movement of buried and arched 
hypocotyl (dimly illuminated from above), traced on horizontal 
glass during 45 hours. Movement of bead of filament magnified 
about 25 times, and here reduced to one-half of original scale. 

yet, only slightly arched hypocotyl had become nearly vertical, 
a glass filament was affixed on two occasions near to the base 
of the basal leg (i.e. the one in connection with the radicle), 
and its movements were traced in darkness on a horizontal 
glass. The result was that long lines were formed running in 
nearly the plane of the vertical arch, due to the early separation 
of the two legs now freed from pressure ; but as the lines were 
zigzag, showing lateral movement, the arch must have been 
circumnutating, whilst it was straightening itself by growth 
along its inner or concave surface. 

A somewhat different method of observation was next fol- 
lowed : as soon as the earth with seeds in a pot began to crack, 
the surface was removed in parts to the depth of -2 inch ; and 
a filament was fixed to the basal leg of a buried and arched 
hypocotyl, just above the summit of the radicle. The cotyle- 
dons were still almost completely enclosed within the much- 
cvacked seed-coats; and these were again covered up with damp 



Chap. I. BRASSICA. 13 

adhesive soil pressed pretty firmly down. The movement of the 
filament was traced (Fig. 3) from 11 a.m. Feb. 5th till 8 a.m. 
Feb. 7th. By this latter period the cotyledons had been dragged 
from beneath the pressed-down earth, but the upper part of the 
hypocotyl still formed nearly a right angle with the lower part. 
The tracing shows that the arched hypocotyl tends at this early 
age to circumnutate irregularly. On the first day the greater 
movement (from right to left in the figure) was not in the plane 
of the vertical and arched hypocotyl, but at right angles to it, 
or in the plane of the two cotyledons, which were still in close 
contact. The basal leg of the arch at the time when the fila- 
ment was affixed to it, was already bowed considerably back- 
wards, or from the cotyledons; had the filament been affixed 
before this bowing occurred, the chief movement would have 
been at right angles to that shown in the figure. A filament 
was attached to anotlier buried hypocotyl of the same age, and 
it moved in a similar general manner, but the line traced was 
not so complex. This hypocotyl became almost straight, and 
the cotyledons were dragged from beneath the ground on the 
evening of the second day. 

Fig. 4. 



**-V 



^ 




Brassica oleracea : circumnutating movement of buried and arched 
hypocotyl, with the two legs of the arch tied together, traced on 
horizontal glass during 33^ hours. Movement of the bead of fila- 
ment magnified about 26 times, and here reduced to one-half origi- 
nal scale. 

Before the above observations were made, some arched hypo- 
cotyls buried at the depth of a quarter of an inch were uncov- 
ered; and in order to prevent the two legs of the arch from 



14 CIRCUMNUTATION OP SEEDLINGS. Chap. I. 

beginning to separate at once, they were tied together with fine 
silk. This was done partly because we wished to ascertain how 
long the hypocotyl, in its arched condition, would continue to 
move, and whether the movement when not masked and dis- 
turbed by the straightening process, indicated circumnutation. 
Firstly, a filament was fixed to the basal leg of an arched hypo- 
cotyl close above the summit of the radicle. The cotyledons 
were still partially enclosed within the seed-coats. The move- 
ment was traced (Fig. 4) from 9.20 a.m. on Dec. 23rd to 6.45 
A.M. on Dec. 25th. No doubt the natural movement was much 
disturbed by the two legs having been tied together ; but we 
see that it was distinctly zigzag, first in one direction and then 
in an almost opposite one. After 3 p.m. on the 24th the arched 
hypocotyl sometimes remained stationary for a considerable 
time, and when moving, moved far slower than before. There- 
fore, on the morning of the 25th, the glass filament was re- 
moved from the base of the basal leg, and was fixed horizon- 
Fig. 5. 




Brassica oleracea : circumnutating movement of the crown of a buried 
and arched hypocotyl, with the two legs tied together, traced on a 
horizontal glass during 23 hours. Movement of the bead of the 
filament magnified about 58 times, and liere reduced to one-hulf 
original scale. 

tally on the summit of the arch, which, from the legs having 
been tied, had grown broad and almost flat. The movement 
was now traced during 23 hours (Fig. 5), and we see that the 



Chap. I. BEASSICA. 15 

course was still zigzag, which indicates a tendency to circum- 
nutation. The base of the basal leg by this time had almost 
completely ceased to move. 

As soon as the cotyledons have been naturally dragged from 
beneath the ground, and the hypocotyl has straightened itself 
by growth along the inner or concave surface, there is nothing 
to interfere with the free movements of the parts ; and the cir- 
cumnutation now becomes much more regular and clearly dis- 
played, as shown in the following cases: — A seedling was 



Fig. 6. 




Brassica oleracea : conjoint circuranutation of the hypocotyl and coty- 
ledons during 10 hours 45 minutes. Figure here reduced to oue- 
half original scale. 

placed in front and near a north-east window with a line join- 
ing the two cotyledons parallel to the window. It was thus 
left the whole day so as to accommodate itself to the light. 
On the following morning a filament was fixed to the midrib of 
the larger and taller cotyledon (which enfolds the other and 
smaller one, whilst still within the seed), and a mark being 
placed close behind, the movement of the whole plant, that is, 
of the hypocotyl and cotyledon, was traced greatly magnified 
on a vertical glass. At first the plant bent so much towards the 
light that it was useless to attempt to trace the movement ; but 
at 10 A.M. heliotropism almost wholly ceased and the first dot 



16 



CIRCUMNUTATION OF SEEDLINGS. Chap. I. 



was made on the glass. The last was made at 8.45 p.m. ; seven- 
teen dots being altogether made in this interval of 10 h. 45 m. 
(see Fig. 6). It should be noticed that when I looked shortly 
after 4 p.m. the bead was pointing off the glass, but it came 
on again at 5.30 p.m., and the course during this interval of 1 h. 
30 m. has been filled up by imagination, but cannot be far from 
correct. The bead moved seveii^ times from side to side, and 
thus described d^ ellipses in lOf h. ; each being completed on 
an average in 3 h. 4 m. 

On the previous day another seedling had been observed 
under similar conditions, excepting that the plant was so placed 

Fig. 7. 




Brassica oleracea : conjoint circumnutation of the hypocotyl and coty- 
ledons, from 10.50 A.M. to 8 a.m. on the following morning. 
Tracing made on a vertical glass. 

that a line joining the two cotyledons pointed towards the win- 
dow ; and the filament was attached to the smaller cotyledon 
on the side furthest from the window. Moreover, the plant 



Chap. I. 



BRASSICA. 



17 




was now for the first time placed in this position. The cotyle- 
dons bowed themselves greatly towards the light from 8 to 
10.50 A.M., when the first dot was made (Fig. 7). During the 
next 12 hours the bead 

swept obliquely up and Fig. 8. 

down 8 times and de- 
scribed 4 figures repre- 
senting ellipses; so that 
it travelled at nearly the 
same rate as in the pre- 
vious case. During the 
night it moved upwards, 
owing to the sleep-move- 
ment of the cotyledons, 
and continued to move 
in the same direction till 
9 A.M. on the following 
morning; but this latter 
movement would not 
have occurred with seed- 
lings under their natural 
conditions fully exposed 
to the light. 

By 9.25 A.M. on this second day the same cotyledon had 
begun to fall, and a dot was made on a fresh glass. The move- 
ment was traced until 5. 30 p.m. as shown in (Fig. 8), which is 
given, because the course followed was much more irregular 
than on the two previous occasions. During these 8 hours the 
bead changed its course greatly 10 times. The upward move- 
ment of the cotyledon during the afternoon and early part of 
the night is here plainly shown. 

As the filaments were fixed in the three last cases to one of 
the cotyledons, and as the hypocotyl was left free, the tracings 
show the movement of both organs conjoined; and we now 
wished to ascertain whether both circumnutated. Filaments 
were therefore fixed horizontally to two hypocotyls close be- 
neath the petioles of their cotyledons. These seedlings had 
stood for two days in the same position before a north-east 
window. In the morning, ilp to about 11 a.m., they moved in 



Brassica oleracea : conjoint circumnu- 
tation of the hypocotyl and cotyle- 
dons during 8 hours. Figure here 
reduced to one-third of the original 
scale, as traced on a vertical glass. 



18 



CIRCUMNUTATION OF SEEDLINGS. Chap. I. 



zigzag lines towards the light ; and at night they again became 
almost upright through apogeotropism. After about 11 a.m. 
they moved a little back from the light, often crossing and 

recrossing their former path 
in zigzag lines. The sky on 
this day varied much in 
brightness, and these obser- 
vations merely proved that 
the hypocotyls were continu- 
ally moving in a manner 
resembling circumnutation. 
On a previous day which 
was uniformly cloudy, a hy- 
pocotyl was firmly secured 
to a little stick, and a fila- 
ment was fixed to the larger 
of the two cotyledons, and 
its movement was traced on 
a vertical glass , It fell great- 
ly from 8.53 a.m., when the 
first dot was made, till 10.55 
A.M. ; it then rose greatly 
until 12.17 P.M. Afterwards 
it fell a little and made a 
loop, but by 2.22 p.m. it had 
risen a little and continued 
rising till 9.23 p.m., when it 
made another loop, and at 
10.30 P.M. was again rising. 
These observations show that 
the cotyledons move verti- 
cally up and down all day 
long, and as there was some 
slight lateral movement, they 
circumnutated. 

The cabbage was one of 
the first plants, the seedlings of which were observed by us, and 
we did not then know how far the circumnutation of the differ- 
ent parts was affected by light. Young seedlings were therefore 




Brassica oleracea : circumnutation of 
hypocotyl, in darkness, traced on 
a liorizontal glass, by means of a 
filament with a bead fixed across 
its summit, between 9.15 A.M. and 
8.30 A.M. on the following morn- 
ing. Figure here reduced to one- 
half of original scale. 



Chap. I. 



BRASSICA. 



19 



Fig. 10. 



kept in complete darkness except for a minute or two during 
each observation, when they were illuminated by a small wax 
taper held almost vertically above them. During the first day 
the hypocotyl of one changed its course 13 times (see Fig. 9); 
and it deserves notice that the longer axes of the figures 
described often cross one another at right or nearly right 
angles. Another seedling was observed in the same manner, 
but it was much older, for it had formed a true leaf a quarter 
of an inch in length, and the hypocotyl was If inch in height. 
The figure traced was a very complex one, 
though the movement was not so great in 
extent as in the last case. 

The hypocotyl of another seedling of 
the same age "was secured to a little stick, 
and a filament having been fixed to the 
midrib of one of the cotyledons, the move- 
ment of the bead was traced during the 

14 h. 15 m. (see Fig. 10) in darkness. It 
should be noted that the chief movement 
of the cotyledons, namely, up and down, 
would be shown on a horizontal glass- 
plate only by the lines in the direction of 
the midrib (that is, up and down, as Fig. 
10 here stands) being a little lengthened 
or shortened ; whereas any lateral move- 
ment would be well exhibited. The pres- 
ent tracing shows that the cotyledon did 
thus move laterally (that is, from side to 
side in the tracing) 12 times in the 14 h. 

15 m. of observation. Therefore the coty- 
ledons certainly circumnutated, though the chief movement 
was up and down in a vertical plane. 

Rate of movement. — The movements of the hypocotyls and 
cotyledons of seedling cabbages of different ages have now 
been sufficiently illustrated. With respect to the rate, seedlings 
were placed under the microscope with the stage removed, and 
with a micrometer eye-piece so adjusted that each division 
equalled ^^^ inch ; the plants were illuminated by light passing 
through a solution of bichromate of potassium so as to elimi- 




Brassica oleracea : cir- 
cumnutation of a 
cotyledon, tlie hypo- 
cotyl having been 
secured to a stick, 
traced on a horizon- 
tal glass, in dark- 
ness, from 8.15 A.M. 
to 10.30 P.M. Move- 
ment of the bead of 
the filament magni- 
fied 13 times. 



20 CmCUMNUTATION OF SEEDLINGS. Chap. T. 

nate heliotropism. Under these circumstances it was interest- 
ing to observe how rapidly the circumnutating apex of a coty- 
ledon passed across the divisions of the micrometer. Whilst 
travelling in any direction the apex generally oscillated back- 
wards and forwards to the extent of -j-i-g- and sometimes of 
nearly ^t^- of an inch. These oscillations were quite different 
from the trembling caused by any disturbance in the same room 
or by the shutting of a distant door. The first seedling ob- 
served was nearly two inches in height and had been etiolated 
by having been grown in darkness. The tip of the cotyledon 
passed across 10 divisions of the micrometer, that is, ^-^ of an 
inch, in 6 m. 40 s. Short glass filaments were then fixed verti- 
cally to the hypocotyls of several seedlings so as to project a 
little above the cotyledons, thus exaggerating the rate of move- 
ment ; but only a few of the observations thus made are worth 
giving. The most remarkable fact was the oscillatory move- 
ment above described, and the difference of rate at which the 
point crossed the divisions of the micrometer, after short inter- 
vals of time. For instance, a tall not-etiolated seedling had 
been kept for 14 h. in darkness ; it was exposed before a north- 
east window for only two or three minutes whilst a glass fila- 
ment was fixed vertically to the hypocotyl; it was then again 
placed in darkness for half an hour and afterwards observed by 
light passing through bichromate of potassium. The point, 
oscillating as usual, crossed five divisions of the micrometer 
(i. e. y^o inch) in 1 m. 30 s. The seedling was then left in 
darkness for an hour, and now it required 3 m. 6 s. to cross one 
division, that is, 15 m. 30 s. to have crossed five divisions. An- 
other seedling, after being occasionally observed in the back 
part of a northern room with a very dull light, and left in 
complete darkness for intervals of half an hour, crossed five 
divisions in 5 m. in the direction of the window, so that we 
concluded that the movement was heliotropic. But this 
was probably not the case, for it was placed close to a north- 
east window and left there for 25 m., after which time, in- 
stead of moving still more quickly towards the light, as 
might have been expected, it travelled only at the rate of 
12 ra. 30 s. for five divisions. It was then again left in com- 
plete darkness for 1 h., and the point now travelled in the 



Chap. I. GITHAGO. 21 

same direction as before, but at the rate of 3 m. 18 s. for five 
divisions. 

We shall have to recur to the cotyledons of the cabbage in 
a future chapter, w^hen we treat of their sleep-movements. The 
circumnutation, also, of the leaves of fully-developed plants 
will hereafter be described. 

Githago segetum (Caryophyllese). — A young seedling v^as 
dimly illuminated from above, and the circumnutation of the 
hypocotyl was observed during 28 h., as shown in Fig. 11. It 

Fig. 11. 





Githago segetum : circnmnutation of hypocotyl, traced on a horizontal 
glass, by means of a filament fixed transversely across its summit, 
from 8.15 a.m. to 12.15 P.M. on the following day. Movement of 
bead of filament magnified about 13 times, here reduced to oue-half 
the original scale. 

moved in all directions ; the lines from right and to left in the 
figure being parallel to the blades of the cotyledons. The 
actual distance travelled from side to side by the summit of the 
hypocotyl was about '2 of an inch; but it was impossible to 
be accurate on this head, as the more obliquely the plant was 
viewed, after it had moved for some time, the more the dis- 
tances were exaggerated. 

We endeavoured to observe the circumnutation of the coty- 
ledons, but as they close together unless kept exposed to a 
moderately bright light, and as the hypocotyl is extremely 
heliotropic, the necessary arrangements were too troublesome. 
We shall recur to the nocturnal or sleep-movements of the coty- 
ledons in a future chapter. 



22 



CIRCUMNUTATION OF SEEDLINGS. Chap. I. 



Fig. 12. 




Oossypium (var. Nankin cotton) (Malvaceae). — The circum- 
nutation of a hypocotyl was observed in the hot-house, but the 
movement was so much exaggerated that the bead twice passed 
for a time out of view. It was, however, 
manifest that two somewhat irregular 
ellipses were nearly completed in 9 h. 
Another seedling, 1^ in. in height, was 
then observed during 23 h. ; but the ob- 
servations were not made at sufficiently 
short intervals, as shown by the few dots 
in Fig. 12, and the tracing was not now 
suffi.ciently enlarged. Nevertheless there 
could be no doubt about the circumnu- 
tation of the hypocotyl, which described 
across in 12 h. a figure representing three irreg- 
ular ellipses of unequal sizes. 

The cotyledons are in constant move- 
ment up and down during the whole 
day, and as they offer the unusual case of 
moving downwards late in the evening and in the early part of 
the night, many observations were made on them. A filament 
was fixed along the middle of one, and its movement traced on 
a vertical glass ; but the tracing is not given, as the hypocotyl 
was not secured, so that it was impossible to distinguish clearly 
between its movement and that of the cotyledon. The coty- 
ledons rose from 10.30 a.m. to about 3 p.m. ; they then sank 
till 10 P.M., rising, however, greatly in the latter part of the 
night. The angles above the horizon of which the cotyledons 
of another seedling stood at different hours is recorded in the 
following short table : — 



Gossypium : circumnu- 
tation of hypocotyl, 
traced on a horizon- 
tal glass, from 10.30 
A.M. to 9.30 A.M. on 
following morning, 
by means of a fila- 
ment fixed 
its summit. Move- 
ment of bead of fila- 
ment magnified about 
twice ; seedling illu- 
minated from above. 



Oct. 20 2.50 P.M. . . 






25° above horizon 


4.20 " . . 






22° 


5.20 " - . . 






15° 


" 10.40 " . . 






go 


Oct. 21 8.40 A.M. . . 






28° 


" 11.15 " . . 






35° 


9.11p.m. . . 






10° below horizon 



The position of the two cotyledons was roughly sketched at 
various hours with the same general result. 

In the following summer, the hypocotyl of a fourth seedling 



Chap. I. OXALIS. 23 

was secured to a little stick, and a glass filament with triangles 
of paper having been fixed to one of the cotyledons, its move- 
ments were traced on a vertical glass under a double skylight 
in the house. The first dot was made at 4.20 p.m. June 20th ; 
and the cotyledon fell till 10.15 p.m. in a nearly straight line. 
Just past midnight it was found a little lower and somewhat 
to one side. By the early morning, at 3.45 a.m., it had risen 
greatly, but by 6.20 a.m. had fallen a little. During the whole 
of this day (21st) it fell in a slightly zigzag line, but its normal 
course was disturbed by the want of sufficient illumination, for 
during the night it rose only a little, and travelled irregularly 
during the whole of the following day and night of June 22nd. 
The ascending and descending lines traced during the three 
days did not coincide, so that the movement was one of circum- 
nutation. This seedling was then taken back to the hot-house, 
and after five days was inspected at 10 p.m., when the cotyle- 
dons were found hanging so nearly vertically down, that they 
might justly be said to have been asleep. On the following 
morning they had resumed their usual horizontal position. 

Oxalis rosea (Oxalidese). — The hypocotyl was secured to a 
little stick, and an extremely thin glass filament, with two tri- 
angles of paper, was attached to one of the cotyledons, which 
was '15 inch in length. In this and the following species the 
end of the petiole, where united to the blade, is developed into 
a pulvinus. The apex of the cotyledon stood only 5 inches 
from the vertical glass, so that its movement was not greatly 
exaggerated as long as it remained nearly horizontal ; but in 
the course of the day it both rose considerably above and fell 
beneath a horizontal position, and then of course the movement 
was much exaggerated. In Fig. 13 its course is shown from 
6.45 A.M. on June 17th, to 7.40 a.m. on the following morning; 
and we see that during the daytime, in the course of 11 h. 15 
m., it travelled thrice down and twice up. After 5.45 p.m. it 
moved rapidly downwards, and in an hour or two depended 
vertically; it thus remained all night asleep. This position 
could not be represented on the vertical glass nor in the figure 
here given. By 6.40 a.m. on the following morning (18th) both 
cotyledons had risen greatly, and they continued to rise until 8 
A.M., when they stood almost horizontally. Their movement 
3 



24 



CIRCUMNUTATION OF SEEDLINGS. Chap. I. 



Fig. 13. 



was traced during the whole of this day and until the next 
morning; but a tracing is not given, as it was closely similar to 

Fig. 13, excepting that the lines 
were more zigzag. The coty- 
ledons moved 7 times, either up- 
wards or downwards; and at 
about 4 P.M. the great nocturnal 
sinking movement commenced. 
Another seedling was ob- 
served in a similar manner dur- 
ing nearly 24 h., but with the 
difference that the hypocotyl 
was left free. The movement 
also was less magnified. Be- 
tween 8.12 A.M. and 5 p.m. on 
the 18th, the apex of the coty- 
ledon moved 7 times upwards 
or downwards (Fig. 14). The 
nocturnal sinking movement, 
which is merely a great increase 
of one of the diurnal oscillations, 
commenced about 4 p.m. 

Oxalis Valdimana. — This spe- 
cies is interesting, as the coty- 
ledons rise perpendicularly up- 
wards at night so as to come 
into close contact, instead of 
sinking vertically downwards, 
as in the case of 0. rosea. A 
glass filament was fixed to a 
cotyledon, "17 of an inch in 
length, and the hypocotyl was 
left free. On the first day the 
// seedling was placed too far from 

s the vertical glass; so that the 

^ ,. . , ,. ^ tracing was enormously exag- 

Oxahs rosea: circumnutation of t t , . Vj 

cotyledons, the hypocotyl being gerated and the movement could 

secured to a stick ; illuminated ^ot be traced when the coty- 

from above. Figure here given , ^ .,, , , 

one-half of original scale. ledon either rose or sank much ; 




a^ao'a.m. 



u 

i I 

! 



Chap. I. 



OXALIS. 



25 



but it was clearly seen that the cotyledons rose thrice and fell 
twice between 8.15 a.m. and 4.15 p.m. Early on the following 
morning (June 19th) the apex of a cotyledon was placed only 
1| inch'from the vertical glass. At 6.40 a.m. it stood horizon- 



Fig. 14. 



Fig. 15. 



e°2B' 



j^pW 



OxaKs rosea : conjoint circumnutation 
of the cotyledons and hypocotyl, 
traced from 8.12 a.m. on June 18th 
to 7.30 A.M. 19th. The apex of the 
cotyledon stood only 3| inches from 
the vertical glass. Figure here given 
one-half of original scale. 



S^^S' 



\l°2&^$.nu 




IQfh 



Oxalis Valdiviana : conjoint 
circumnutation of a coty- 
ledon and the hypocotyl, 
traced on vertical glass, 
during 24 hours. Figure 
here given one - half of 
original scale : seedling il- 
luminated from above. 



tally; it then fell till 8.35, and then rose. Altogether in the 
course of 12 h. it rose thrice and fell thrice, as may be seen in 
Fig. 15. The great nocturnal rise of the cotyledons usually 
commences about 4 or 5 p.m., and on the following morning 
they are expanded or stand horizontally at about 6.30 a.m. 



26 CIRCUMNUTATION OF SEEDLINGS. Chap. I. 

In the present instance, however, the great nocturnal rise did 
not commence till 7 p.m. ; but this was due to the hypocotyl 
having from some unknown cause temporarily bent to the left 
side, as is shown in the tracing. To ascertain positively that 
the hypocotyl circumnutated, a mark was placed at 8.15 p.m. 
behind the two now closed and vertical cotyledons ; and the 
movement of a glass filament fixed upright to the top of the 
hypocotyl was traced until 10.40 p.m. During this time it 
moved from side to side, as well as backwards and forwards, 
plainly showing circumnutation ; but the movement was small 
in extent. Therefore Fig. 15 represents fairly well the move- 
ments of the cotyledons alone, with the exception of the one 
great afternoon curvature to the left. 

Oxalis carniculata (var. cwprea). — The cotyledons rise at 
night to a variable degree above the horizon, generally about 
45° : those on some seedlings between 3 and 5 days old were 
found to be in continued movement all day long; but the 
movements were more simple than in the last two species. 
This may have partly resulted from their not being sufficiently 
illuminated whilst being observed, as was shown by their not 
beginning to rise until very late in the evening. 

Oxalis {BiopJiytum) sensitiva. — The cotyledons are highly 
remarkable from the amplitude and rapidity of their move- 
ments during the day. The angles at which they stood above 
or beneath the horizon were measured at short intervals of 
time; and we regret that their course was not traced during 
the whole day. We will give only a few of the measurements, 
which were made whilst the seedlings were exposed to a tem- 
perature of 22^° to 241° C. One cotyledon rose 70° in 11 m. ; 
another, on a distinct seedling, fell 80° in 12 m. Immediately 
before this latter fall the same cotyledon had risen from a ver- 
tically downward to a vertically upward position in 1 h. 48 m., 
and had therefore passed through 180° in under 2 h. We have 
met with no other instance of a circumnutating movement of 
such great amplitude as 180°; nor of such rapidity of move- 
ment as the passage through 80° in 12 m. The cotyledons of 
this plant sleep at night by rising vertically and coming into 
close contact. This upward movement differs from one of the 
great diurnal oscillations above described only by the position 



Chap. I. 



OXALIS. 



27 



Fig. 16. 



being permanent during the night and by its periodicity, as it 
always commences late in the evening. 

TropoBolum minus (?) (var. Tom Thumb) (Tropseoleae). — The 
cotyledons are hypogean, or never rise above the ground. By 
removing the soil a buried epicotyl or plumule was found, with 
its summit arched abruptly downwards, like the arched hypo- 
cotyl of the cabbage previously 
described. A glass filament with 
a bead at its end was aflixed to 
the basal half or leg, just above 
the hypogean cotyledons, which 
were again almost surrounded 
by loose earth. The tracing 
(Fig. 16) shows the course of the 
bead during 11 h. After the 
last dot given in the figure, the 
bead moved to a great distance, 
and finally off the glass, in the 
direction indicated by the broken 
line. This great movement, due 
to increased growth along the 
concave surface of the arch, was 
caused by the basal leg bending 
backwards from the upper part, 
that is, in a direction opposite to 
the dependent tip, in the same 
manner as occurred with the 
hypocotyl of the cabbage. An- 
other buried and arched epicotyl was observed in the same 
manner, excepting that the two legs of the arch were tied 
together with fine silk for the sake of preventing the great 
movement just mentioned. It moved, however, in the even- 
ing in the same direction as before, but the line followed was 
not so straight. During the morning the tied arch moved 
in an irregularly circular, strongly zigzag course, and to a 
greater distance than in the previous case, as was shown in a 
tracing, magnified 18 times. The movements of a young plant 
bearing a few leaves and of a mature plant, will hereafter be 
described. 




Tropseolum minus (?) : circum- 
nutation of buried and arched 
epicotyl, traced on a horizon- 
tal glass, from 9.20 a.m. to 
8.15 P.M. Movement of bead 
of filament magnified 27 times. 



■28 



CIRCUMNUTATION OF SEEDLINGS. Chap. I. 



Cit/rns aurantinm (Orange) (Aurantiacese). — The cotyledons 
are hypogean. The circumnutation of an epicotyl, which at the 
close of our observations was '59 of an inch (15 mm.) in height 
above the ground, is shown in the annexed figure (Fig. 17) as 
observed during a period of 44 h. 40 m. 



Fig. 17. 




Citrus aurantium : circumnutation of epicotyl with a filament fixed 
transversely near its apex, traced on a horizontal glass, from 12.13 
P.M. on Feb. 20th to 8.55 A.M. on 22nd. The movement of the 
bead of the filament was at first magnified 21 times, or 10^, in fig- 
ure here given, and afterwards 36 times, or 18 as here given ; seed- 
ling illuminated from above. 



^sculus hippocastanum (Hippocastaneae). — Germinating 
seeds were placed in a tin box, kept moist internally, with a 
sloping bank of damp argillaceous sand, on which four smoked 
glass-plates rested, inclined at angles of 70° and 65° with the 
horizon. The tips of the radicles were placed so as just to 
touch the upper end of the glass-plates, and, as they grew 
downwards they pressed lightly, owing to geotropism, on the 
smoked surfaces, and left tracks of their course. In the mid- 
dle part of each track the glass was swept clean, but the mar- 
gins were much blurred and irregular. Copies of two of these 
tracks (all four being nearly alike) were made on tracing paper 
placed over the glass-plates after they had been varnished ; and 
they are as exact as possible, considering the nature of the 
margins (Fig. 18). They suffice to show that there was some 
lateral, almost serpentine movement, and that the tips in their 



Chap. I. 



VICIA. 



29 




Msculus hippocastanum : out- 
lines of tracks left on in- 
clined glass-plates by tips 
of radicles. In A the plate 
was inclined at 70° with 
the horizon, and the radicle 
was 1'9 inch in length, and 
"23 inch in diameter at base. 
In B the plate was inclined 
65° with the horizon, and 
the radicle was a trifle 
larger. 



downward course pressed with unequal force on the plates, as 
the tracks varied in breadth. The more perfectly serpentine 
tracks made by the radicles of Pha- 
seolus multijiorus and Vicia fal)a 
(presently to be described), render 
it almost certain that the radicles 
of the present plant circumnutated. 
Phaseolus multijiorus (Legumi- 
nosse). — Four smoked glass-plates 
were arranged in the same manner 
as described under ^sculus, and 
the tracks left by the tips of four 
radicles of the present plant, whilst 
growing downwards, were photo- 
graphed as transparent objects. 
Three of them are here exactly 
copied (Fig. 19). Their serpentine 
courses show that the tips moved 
regularly from side to side; they 
also pressed alternately with greater or less force on the plates, 
sometimes rising up and leaving them altogether for a very 
short distance ; but this was better seen on the original plates 
than in the copies. These radicles therefore were continually 
moving in all directions — that 
is, they circumnutated. The 
distance between the extreme 
right and left positions of the 
radicle A, in its lateral move- 
ment, was 3 mm. , as ascertained 
by measurement with an eye- 
piece micrometer. 

Vicia faba (Common Bean) 
(Leguminosas). — Radicle. — Some 
beans were allowed to germinate 
on bare sand, and after one had 
protruded its radicle to a length 
of -3 of an inch, it was turned 
upside down, so that the radicle, which was kept in damp air, 
now stood upright. A filament, nearly an inch in length, was 



Fig. 19. 



^ 



Phaseolus multijiorus : tracks left 
on inclined smoked glass-plates 
by tips of radicles in growing 
downwards. A and C, plates 
inclined at 60°, B inclined at 
68° with the horizon. 



30 CIRCUMNUTATION OF SEEDLINGS. Chap. I. 

affixed obliquely near its tip ; and the movement of the termi- 
nal bead was traced from 8.30 a.m. to 10.30 p.m., as shown 
in Fig. 18. The radicle at first changed its course twice 
abruptly, then made a small loop and then a larger zigzag 
curve. During the night and till 11 a.m. on the following 
morning, the bead moved to a great distance in a nearly 
straight line, in the direction indicated by the broken line in 
the figure. This resulted from the tip bending quickly down- 
wards, as it had now become much declined, and had thus 
gained a position highly favourable for the action of geot- 
ropism. 

We next experimented on nearly a score of radicles by 
allowing them to grow downwards over inclined plates of 
smoked glass, in exactly the same manner as with ^sculus and 
Phaseolus. Some of the plates were inclined only a few de- 
grees beneath the horizon, but most of them between 60° and 
75°. In the latter cases the radicles in growing downwards 
were deflected only a little from the direction which they had 



Vicia faba : circumnutation of a i-adicle, at first pointing vertically 
upwards, kept in darkness, traced on a horizontal glass, during 14 
horn's. Movement of bead of filament magnified 23 times, here 
reduced to one-half of original scale. 

followed while germinating in sawdust, and they pressed lightly 
on the glass-plates (Fig. 21). Five of the most distinct tracks 
are here copied, and they are all slightly sinuous, showing 
circumnutation. Moreover, a close examination of almost 
every one of the tracks clearly showed that the tips in their 
downward course had alternately pressed with greater or less 



Chap. I. EPICOTYL. 31 

force on the plates, and had sometimes risen up so as nearly to 
leave them for short intervals. The distance between the ex- 
treme right and left positions of the radicle A was 0-7 mm., 
ascertained in the same manner as in the case of Phaseolus. 

Fig. 21. 




C. D. E. 

Vicia faha : tracks left on inclined smoked glass-plates, by tips of 
radicles in growing downwards. Plate C was inclined at 63°, 
plates A and D at 71°, plate B at 75°, and plate E at a few degrees 
beneath the horizon. 

Epicotyl. — ^At the point where the radicle had protruded 
from a bean laid on its side, a flattened solid lump projected 
•1 of an inch, in the same horizontal plane with the bean. This 
protuberance consisted of the convex summit of the arched 
epicotyl ; and as it became developed the two legs of the arch 
curved themselves laterally upwards, owing to apogeotropism, 
at such a rate that the arch stood highly inclined after 14 h., 
and vertically in 48 h. A filament was fixed to the crown of 
the protuberance before any arch was visible, but the basal half 
grew so quickly that on the second morning the end of the fila- 
ment was bowed greatly downwards. It was, therefore, re- 
moved and fixed lower down. The line traced during these 
two days extended in the same general direction, and was in 
parts nearly straight, and in others plainly zigzag, thus giving 
some evidence of circumnutation. 

As the arched epicotyl, in whatever position it may be 
placed, bends quickly upwards through apogeotropism, and as 
the two legs tend at a very early age to separate from one 
another, as soon as they are relieved from the pressure of the 




32 CIRCUMNUTATION OF SEEDLINGS. Chap. I. 

surrounding earth, it was difficult to ascertain positively whether 
the epicotyl, whilst remaining arched, circumnutated. There- 
fore, some rather deeply buried beans were uncovered, and the 
two legs of the arches were tied together, as had been done 
with the epicotyl of Tropseolum and the hypocotyl of the Cab- 
bage. The movements of 
Fig. 22. the tied arches were traced 

in the usual manner on two 
occasions during three days. 
But the tracings made un- 
der such unnatural condi- 
tions are not worth giving ; 
and it need only be said that 
the lines were decidedly zig- 
zag, and that small loops 
were occasionally formed. 
"We may therefore conclude 
Viciafaba : circumnutation of young that the epicotyl circumnu- 
epicotyl, traced m darkcess dur- ^ "^ 

ing 50 hours on a horizontal glass, tates whilst still arched and 
Movement of bead of filament before it has grown tall 
m^agnined 20 times, here reduced ° 

to one-half of original scale. enough to break through the 

surface of the ground. 

In order to observe the movements of the epicotyl at a 
somewhat more advanced age, a filament was fixed near the 
base of one which was no longer arched, for its upper half now 
formed a right angle with the lower half. This bean had 
germinated on bare damp sand, and the epicotyl began to 
straighten itself much sooner than would have occurred if it 
had been properly planted. The course pursued during 50 h. 
(from 9 A.M. Dec. 26th, to 11 a.m. 28th) is here shown (Fig. 
22) ; and we see that the epicotyl circumnutated during the 
whole time. Its basal part grew so much during the 50 h. that 
the filament at the end of our observations was attached at the 
height of "4 inch above the upper surface of the bean, instead 
of close to it. If the bean had been properly planted, this part 
of the epicotyl would still have been beneath the soil. 

Late in the evening of the 28th, some hours after the above 
observations were completed, the epicotyl had grown much 
straighter, for the upper part now formed a widely open angle 



Chap. I. 



LATHYRUS. 



33 



with the lower part. A filament was fixed to the upright basal 
part, higher up than before, close beneath the lowest scale-like 
process or homologue of a leaf ; and its movement was traced 
during 38 h. (Fig. 33). We here again have plain evidence 
Fig. 23. 




Vicia faba : circumnutation of the same epicotyl as in Fig. 22, a little 
more advanced in age, traced under similar conditions as before, 
from 8.40 A.M. Dec. 28th, to 10.50 a.m. 30th. Movement of bead 
here magnified 20 times. 

of continued circumnutation. Had the bean been properly- 
planted, the part of the epicotyl to which the filament was at- 
tached, the movement of which is here shown, would probably 
have just risen above the surface of the ground. 

Lathyrus nissolia (Leguminosae). — This plant was selected 
^ an abnormal form with grass-like 
Fig. 24. 



for observation from bein 




Lathyrus nissolia : circumnutation of stem of young seedling, traced in 
darkness on a horizontal glass, from 6.45 a.m. Nov. 22nd, to 7 A.M. 
23rd. Movement of end of leaf magnified about 12 times, hero 
reduced to one-half of original scale. 

leaves. The cotyledons are hypogean, and the epicotyl breaks 
through the ground in an arched form. The movements of a 



34 CIRCUMNUTATION OF SEEDLINGS. Chap. I. 

stem, 1-2 inch in height, consisting of three internodes, the 
lower one almost wholly subterranean, and the upper one bear- 
ing a short, narrow leaf, is shown during 24 h. , in Fig. 24. No 
glass filament was employed, but a mark was placed beneath 
the apex of the leaf. The actual length of tlie longer of the 
two ellipses described by the stem was about -14 of an inch. 
On the previous day the chief line of movement was nearly at 
right angles to that shown in the present figure, and it was 
more simple. 

Cassia tora^ (Leguminosae). — A seedling was placed before 
a north-east window ; it bent very little towards it, as the hypo- 
cotyl which was left free was rather old, and therefore not 
highly heliotropic. A filament had been fixed to the midrib of 
one of the cotyledons, and the movement of the whole seedling 
was traced during two days. The circumnutation of the hypo- 
cotyl is quite insignificant compared with that of the cotyle- 
dons. These rise up vertically at night and come into close 
contact ; so that they may be said to sleep. This seedling was 
so old that a very small true leaf had been developed, which at 
night was completely hidden by the closed cotyledons. On 
Sept. 24th, between 8 a.m. and 5 p.m., the cotyledons moved 
five times up and five times down ; they therefore described five 
irregular ellipses in the course of the 9 h. The great nocturnal 
rise commenced about 4.30 p.m. 

On the following morning (Sept. 25th) the movement of 
the same cotyledon was again traced in the same manner dur- 
ing 24 h. ; and a copy of the tracing is here given (Fig. 25). 
The morning was cold, and the window had been accidentally 
left open for a short time, which must have chilled the plant ; 
and this probably prevented it from moving quite as freely as 
on the previous day ; for it rose only four and sank only four 
times during the day, one of the oscillations being very small. 
At 7.10 A.M., when the first dot was made, the cotyledons were 
not fully open or awake; they continued to open till about 9 
A.M., by which time they had sunk a little beneath the horizon: 



* Seeds of this plant, which grew flower well with us; they were 

near the sea-side, were sent to us sent to Kew, and were pronounced 

by Fritz Miiller from S. Brazil. not to be distinguishable from (7. 

The seedlings did not flourish or tora. 



Chap. I. 



CASSIA. 



35 



by 9.30 A.M. they had risen, and then they oscillated up and 
down ; but the upward and downward lines never quite coin- 
cided. At about 4.30 p.m. the great nocturnal rise commenced. 

Fig. 25. 
,.s ram. rJDfa.m^i^^ 





tPttm 

Cassia tora : conjoint circumnutation of cotyledons and hypocotyl, 
traced on vertical glass, from 7.10 A.M. Sept. 25th to 7.30 a.m. 
26th. Figure here given reduced to one-half of original scale. 



At 7 A.M. on the following morning (Sept. 26th) they occupied 
nearly the same level as on the previous morning, as shown in 
the diagram: they then began to open or sink in the usual 
manner. The diagram leads to the belief that the great periodi- 
cal daily rise and fall does not differ essentially, excepting in 
amplitude, from the oscillations during the middle of the day. 



36 CmCUMNUTATION OF SEEDLINGS. Chap. I. 

Lotus Jacobceus (Legurainosse). — The cotyledons of this plant, 
after the few first days of their life, rise so as to stand almost, 
though rarely quite, vertically at night. They continue to act in 
this manner for a long time even after the development of some 
of the true leaves. With seedlings, 3 inches in height, and 
bearing five or six leaves, they rose at night about 45°. They 
continued to act thus for about an additional fortnight. Sub- 
sequently they remained horizontal at night, though still green, 
and at last dropped off. Their rising at night so as to stand 
almost vertically appears to depend largely on temperature; 
for when the seedlings were kept in a cool house, though they 
still continued to grow, the cotyledons did not become vertical 
at night. It is remarkable that the cotyledons do not generally 
rise at night to any conspicuous extent during the first four or 
five days after germination; but the period was extremely vari- 
able with seedlings kept under the same conditions; and many 
were observed. Glass filaments with minute triangles of paper 
were fixed to the cotyledons (1^ mm. in breadth) of two seed- 
lings, only 24 h. old, and the hypocotyl was secured to a stick ; 
their movements greatly magnified were traced, and they cer- 
tainly circumnutated the whole time on a small scale, but they 
did not exhibit any distinct nocturnal and diurnal movement. 
The hypocotyls, when left free, circumnutated over a large space. 

Another and much older seedling, bearing a half-developed 
leaf, had its movements traced in a similar manner during the 
three first days and nights of June ; but seedlings at this age 
appear to be very sensitive to a deficiency of light ; they were 
observed under a rather dim skylight, at a temperature of be- 
tween 16° to 17^° C. ; and apparently, in consequence of these 
conditions, the great daily movement of the cotyledons ceased 
on the third day. During the first two days they began rising 
in the early afternoon in a nearly straight line, until between 
6 and 7 p.m., when they stood vertically. During the latter 
part of the night, or more probably in the early morning, they 
began to fall or open, so that by 6.45 a.m. they stood fully 
expanded and horizontal. They continued to fall slowly for 
some time, and during the second day described a single small 
ellipse, between 9 a.m. and 2 p.m., in addition to the great 
diurnal movement. The course pursued during the whole 24 h. 



Chap. I. MIMOSA. 3Y 

was far less complex than in the foregoing case of Cassia. On 
the third morning they fell very much, and then circumnutated 
on a small scale round the same spot; by 8.20 p.m. they showed 
no tendency to rise at night. Nor did the cotyledons of any of 
the many other seedlings in the same pot rise ; and so it was on 
the following night of June 5th. The pot was then taken back 
into the hot-house, where it was exposed to the sun, and on the 
succeeding night all the cotyledons rose again to a high angle, 
but did not stand quite vertically. On each of the above days 
the line representing the great nocturnal rise did not coincide 
with that of the great diurnal fall, so that narrow ellipses were 
described, as is the usual rule with circumnutating organs. 
The cotyledons are provided with a pulvinus, and its develop- 
ment will hereafter be described. 

Mimosa pudica (Leguminosse). — The cotyledons rise up ver- 
tically at night, so as to close together. Two seedlings were 
observed in the greenhouse (temp. 16° to 17° C. or 63* to 65° F.). 
Their hypocotyls were secured to sticks, and glass filaments 
bearing little triangles of paper were aflBxed to the cotyledons 
of both. Their movements were traced on a vertical glass dur- 
ing 24 h. on November 13th. The pot had stood for some time 
in the same position, and they were chiefly illuminated through 
the glass-roof. The cotyledons of one of these seedlings moved 
downward in the morning till 11.30 a.m., and then rose, moving 
rapidly in the evening until tliey stood vertically, so that in this 
case there was simply a single great daily fall and rise. The 
other seedling behaved rather differently, for it fell in the morn- 
ing until 11.30 A.M., and then rose, but after 12.10 p.m. again 
fell; and the great evening rise did not begin until 1.23 p.m. 
On tlie following morning this cotyledon had fallen greatly 
from its vertical position by 8.15 a.m. Two other seedlings 
(one seven and the other eight days old) had been previously 
observed under unfavourable circumstances, for they had been 
brought into a room and placed before a north-east window, 
where the temperature was between only 56° and 57° F. They 
had, moreover, to be protected from lateral light, and perhaps 
were not sufficiently illuminated. Under these circumstances 
the cotyledons moved simply downwards from 7 a.m. till 2 p.m., 
after which hour and during a large part of the night they con- 



38 CIRCUMNUTATION OF SEEDLINGS. Chap. I. 

tinued to rise. Between 7 and 8 a.m. on the following^ morn- 
ing they fell again; but on this second and likewise on the thud 
day the movements became irregular, and between 3 and 10.30 
P.M. they circumnutated to a small extent about the same spot; 
but they did not rise at night. Nevertheless, on the following 
night they rose as usual. 

Cytisus fragrans (Leguminosae). — Only a few observations 
were made on this plant. The hypocotyl circumnutated to a 
considerable extent, but in a simple manner — namely, for two 
hours in one direction, and then much more slowly back again 
in a zigzag course, almost parallel to the first line, and beyond 
the starting-point. It moved in the same direction all night, 
but next morning began to return. The cotyledons continually 
move both up and down and laterally ; but they do not rise up 
at night in a conspicuous manner. 

Lupinus luteus (Leguminosse). — Seedlings of this plant were 
observed because the cotyledons are so thick (about '08 of an 
inch) that it seemed unlikely that they would move. Our ob- 
servations were not very successful, as the seedlings are strongly 
heliotropic, and their circumnutation could not be accurately 
observed near a north-east window, although they had been 
kept during the previous day in the same position. A seedling 
was then placed in darkness with the hypocotyl secured to a 
stick ; both cotyledons rose a little at first, and then fell dur- 
ing the rest of the day; in the evening between 5 and 6 p.m. 
they moved very slowly: during the night one continued tb 
fall and the other rose, though only a little. The tracing was 
not much magnified, and as the lines were plainly zigzag, the 
cotyledons must have moved a little laterally, that is, they must 
have circumnutated. 

The hypocotyl is rather thick, about -13 of inch ; neverthe- 
less it circumnutated in a complex course, though to a small 
extent. The movement of an old seedling with two true leaves 
partially developed, was observed in the dark. As the move- 
ment was magnified about 100 times it is not trustworthy and 
is not given ; but there could be no doubt that the hypocotyl 
moved in all directions during the day, changing its course 19 
times. The extreme actual distance from side to side through 
which the upper part of the hypocotyl passed in the course of 



Chap. I. CUCURBITA. 39 

14| hours was only -^^ of an inch ; it sometimes travelled at the 
rate of -^^ of an inch in an hour. 

Cucurbita ovifera (Cucurbitaceae). — Ttadicle : a seed which 
had germinated on damp sand was fixed so that the slightly 
curved radicle, which was only -07 inch in length, stood almost 



Fig. 26. 



A- 



Cmurhita ovifera : course followed by a radicle in bending geotropically 
downwards, traced on a horizontal glass, between 11.25 a.m. and 
10.35 P.M. ; the direction during tbe night is indicated by the 
broken line. Movement of bead magnified 14 times. 

vertically upwards, in which position geotropism would act at 
first with little power. A filament was attached near to its 
base, and projected at about an angle of 45° above the horizon. 
The general course followed during the 11 hours of observation 
and during the following night, is shown in the accompanying 
diagram (Fig. 26), and was plainly due to geotropism ; but it 
was also clear that the radicle circumnutated. By the next 
morning the tip had curved so much downwards that the fila- 
ment, instead of projecting at 45° above the horizon, was nearly 
horizontal. Another germinating seed was turned upside down 
and covered with damp sand ; and a filament was fastened to 
the radicle so as to project at an angle of about 50° above the 
horizon; this radicle was '35 of an inch in length and a little 
curved. The course pursued was mainly governed, as in the 
last case, by geotropism, but the line traced during 12 hours 
and magnified as before was more strongly zigzag, again show- 
ing circumnutation. 

Four radicles were allowed to grow downwards over plates 
of smoked glass, inclined at 70° to the horizon, under the same 
conditions as in the cases of ^sculus, Phaseolus, and Vicia, 
Facsimiles are here given (Fig. 27) of two of these tracks; and 
a third short one was almost as plainly serpentine as that at A. 
It was also manifest by a greater or less amount of soot having 
been swept off the glasses, that the tips had pressed alternately 
4 



40 



CIRCUMNUTATION OF SEEDLINGS. Chap. I. 




Cucurbita ovifera : tracks 
left by tips of radicles 
in growing downwards 
over smoked glass- 
plates, inclined at 70° 
to the horizon. 



with greater and less force on them. There must, therefore, 
have been movement in at least two planes at right angles to 
one another. These radicles were so delicate that they rarely- 
had the power to sweep the glasses 
quite clean. One of them had devel- 
oped some lateral or secondary root- 
lets, which projected a few degrees be- 
neath the horizon ; and it is an impor- 
tant fact that three of them left dis- 
tinctly serpentine tracks on the smoked 
surface, showing beyond doubt that 
they had circumnutated like the main 
or primary radicle. But the tracks 
were so slight that they could not be 
traced and copied after the smoked 
surface had been varnished. 

Hyfocotyl. — A seed lying on damp 
sand was firmly fixed by two crossed wires and by its own 
growing radicle. The cotyledons were still enclosed within 
the seed-coats; and the short hypocotyl, between the summit 
of the radicle and the cotyledons, was as yet only slightly 
arched. A filament ("85 of inch 
in length) was attached at an 
angle of 35" above the horizon 
to the side of the arch adjoining 
the cotyledons. This part would 
ultimately form the upper end of 
the hypocotyl, after it had grown 
straight and vertical. Had the 
seed been properly planted, the 
hypocotyl at this stage of growth 
would have been deeply buried 
beneath the surface. The course 
followed by the bead of the fila- 
ment is shown in Fig. 28. The 
chief lines of movement from 
left to right in the figure were parallel to the plane of the two 
united cotyledons and of the flattened seed; and this move- 
ment would aid in dragging them out of the seed-coats, which 



Fig 



,1-..- 




Cucurbita ovifera : circumnuta- 
tion of arched hypocotyl at a 
very early age, traced in dark- 
ness on a horizontal glass, from 
8 A.M. to 10.20 A.M. on the fol- 
lowing day. The movement 
of the bead magnified 20 times, 
here reduced to one -half of 
original scale. 



Chap. I. 



CUCURBITA. 



41 



Fig. 29. 



are held down by a special structure hereafter to be described. 
The movement at right angles to the above lines was due to 
the arched hypocotyl becoming more arched as it increased in 
height. The foregoing observations apply to the leg of the 
arch next to the cotyledons, but the other leg adjoining the 
radicle likewise circumnutated at an equally early age. 

The movement of the same hypocotyl after it had become 
straight and vertical, but with the 
cotyledons only partially expand- 
ed, is shown in Fig. 29. The 
course pursued during 12 h. ap- 
parently represents four and a half 
ellipses or ovals, with the longer 
axis of the first at nearly right 
angles to that of the others. Tlie 
longer axes of all were oblique to 
a line joining the opposite cotyle- 
dons. The actual extreme dis- 
tance from side to side over which 
the summit of the tall hypocotyl 
passed in the course of 12 h. was 
•28 of an inch. The original 
figure was traced on a large scale, 
and from the obliquity of the line 
of view the outer parts of the dia- 
gram are much exaggerated. 

Cotyledons. — On two occasions 
the movements of the cotyledons 

were traced on a vertical glass, and as the ascending and 
descending lines did not quite coincide, very narrow ellipses 
were formed; they therefore circumnutated. Whilst young 
they rise vertically up at night, but their tips always remain 
reflexed; on the following morning they sink down again. 
With a seedling kept in complete darkness they moved in the 
same manner, for they sank from 8.45 a.m. to 4.30 p.m. ; they 
then began to rise and remained close together until 10 p.m., 
when they were last observed. At 7 a.m. on the following 
morning they were as much expanded as at any hour on the 
previous dny. The cotyledons of another young seedling, ex- 




Cucurhita ovifera : circumnuta- 
tion of straight and vertical 
hypocotyl, with filament fast- 
ened transversely across its 
upper end, traced in dark- 
ness on a horizontal glass, 
from 8.30 A.M. to 8.30 P.M. 
The movement of the termi- 
nal bead originally magnified 
about 18 times, here only 4^ 
times. 



42 CmCUMNUTATION OF SEEDLINGS. Chap. T. 

posed to the light, were fully open for the first time on a cer- 
tain day, but were found completely closed at 7 a.m. on the 
following morning. They soon began to expand again, and 
continued doing so till about 5 p.m. ; they then began to rise, 
and by 10.30 p.m. stood vertically and were almost closed. At 
7 A.M. on the third morning they were nearly vertical, and 
again expanded during the day ; on the fourth morning they 
were not closed, yet they opened a little in the course of the 
day and rose a little on the following night. By this time a 
minute true leaf had become developed. Another seedling, 
still older, bearing a well-developed leaf, had a sharp rigid 
filament aflBxed to one of its cotyledons (85 mm. in length), 
which recorded its own movements on a revolving drum with 
smoked paper. The observations were made in the hot-house, 
where the plant had lived, so that there was no change in 
temperature or light. The record commenced at 11 a.m. on 
February 18th; and from this hour till 3 p.m. the cotyledon 
fell; it then rose rapidly till 9 p.m., then very gradually till 
3 A.M. February 19th, after which hour it sank gradually till 
4.30 P.M.; but the downward movement was interrupted by 
one slight rise or oscillation about 1.30 p.m. After 4.30 p.m. 
(19th) the cotyledon rose till 1 a.m. (in the night of February 
20th) and then sank very gradually till 9.30 a.m., when our 
observations ceased. The amount of movement was greater on 
the 18th than on the 19th or on the morning of the 20th. 

Cucurbita aurantia. — An arched hypocotyl was found buried 
a little beneath the surface of the soil ; and in order to prevent 
it straightening itself quickly, when relieved from the surround- 
ing pressure of the soil, the two legs of the arch were tied 
together. The seed was then lightly covered with loose damp 
earth. A filament with a bead at the end was affixed to the 
basal leg, the movements of which were observed during two 
days in the usual manner. On the first day the arch moved in 
a zigzag line towards the side of the basal leg. On the next 
day, by which time the dependent cotyledons had been dragged 
above the surface of the soil, the tied arch changed its course 
greatly nine times in the course of 14| h. It swept a large, 
extremely irregular, circular figure, returning at night to nearly 
the same spot wlience it had started early in the morning. The 



Chap. I. 



CUCURBITA. 



43 



10° 35' p.m. 



6°3B'a.m. 



line was so strongly zigzag that it apparently represented five 
ellipses, with their longer axes pointing in various directions. 
"With respect to the periodical 
movements of the cotyledons, ^, 

those of several young seedlings / '. 

formed together at 4 p.m. an ' '*^'"'"'- 

angle of about 60°, and at 10 
P.M. their lower parts stood ver- 
tically and were in contact ; their 
tips, however, as is usual in the 
genus, were permanently reflexed. 
These cotyledons, at 7 a.m. on the 
following morning, were again 
well expanded. 

Lagenaria vulgaris (var. minia- 
ture Bottle - gourd) (Cucurbita- 
cese). — A seedling opened its coty- 
ledons, the movements of which 
were alone observed, slightly on 
June 27th, and closed them at 
night: next day, at noon (28th), 
they included an angle of 53°, 
and at 10 p.m. they were in close 
contact, so that each had risen 
26i°. At noon, on the 29th, they 
included an angle of 118°, and at 
10 p.m. an angle of 54°, so each 
had risen 32°. On the following 
day they were still more open, 
and the nocturnal rise was greater, 
but the angles were not meas- 
ured. Two other seedlings were 
observed, and behaved during 
three days in a closely similar 
manner. The cotyledons, there- 
fore, open more and more on each 
succeeding day, and rise each night about 30"; consequently 
during the first two nights of their life they stand vertically 
and come into contact. 



1?3& ^, 



lD°a.i]ii 



Lagenaria vulgaris : circumnu- 
tation of a cotyledon, Ih inch 
in length, apex only 4| inches 
from the vertical glass, on 
which its movements were 
traced from 7.35 a.m. July 
11th to 9.5 A.M. on the 14th. 
Figure here given reduced to 
one-third of original scale. 



44 CIRCUMNUTATION OP SEEDLINGS. Chap. I. 

In order to ascertain more accurately the nature of these 
movements, the hypocotyl of a seedling, with its cotyledons 
well expanded, was secured to a little stick, and a filament with 
triangles of paper was affixed to one of the cotyledons. The 
observations were made under a rather dim skylight, and the 
temperature during the whole time was between 17^° to 18° C. 
(63° to 65° F.). Had the temperature been higher and the 
light brighter, the movements would probably have been greater. 
On July nth (see Fig. 30), the cotyledon fell from 7.35 a.m. 
till 10 A.M. ; it then rose (rapidly after 4 p.m.) till it stood quite 
vertically at 8.40 p.m. During the early morning of the next 
day (12th) it fell, and continued to fall till 8 a.m., after which 
hour it rose, then fell, and again rose, so that by 10.35 p.m. it 
stood much higher than it did in the morning, but was not ver- 
tical as on the preceding night. During the following early 
morning and whole day (13th) it fell and circumnutated, but 
had not risen when observed late in the evening ; and this was 
probably due to the deficiency of heat or light, or of both. We 
thus see that the cotyledons became more widely open at noon 
on each succeeding day ; and that they rose considerably each 
night, though not acquiring a vertical position, except during 
the first two nights. 

Cucumis dudaim (CucurbitacesB). — Two seedlings had opened 
their cotyledons for the first time during the day, — one to the 
extent of 90° and the other rather more; they remained in 
nearly the same position until 10.40 p.m. ; but by 7 a.m. on the 
following morning the one which had been previously open to 
the extent of 90° had its cotyledons vertical and completely 
shut; the other seedling had them nearly shut. Later in the 
morning they opened in the ordinary manner. It appears there- 
fore that the cotyledons of this plant close and open at some- 
what different periods from those of the foregoing species of 
the allied genera of Cucurbita and Lagenaria. 

Opuntia hasilaris (Cactese). — A seedling was carefully ob- 
served, because considering its appearance and the nature of the 
mature plant, it seemed very unlikely that either the hypocotyl 
or cotyledons would circumnutate to an appreciable extent. 
The cotyledons were w^cll developed, being -9 of an inch in 
length, '22 in breadth, and 'IS in thickness. The almost cylin- 



Chap. I. 



HELIANTHUS. 



45 



Fig. 31. 





r 



drical hypocotyl, now bearing a minute spinous bud on its sum- 
mit, was only -45 of an inch in height, and '19 in diameter. 
The tracing (Fig. 31) shows the combined movement of the 
hypocotyl and of one of the cotyledons, from 4.45 p.m. on May 
28th to 11 A.M. on the 31st. 
On the 29th a nearly perfect 
ellipse was completed. On the 
30th the hypocotyl moved, from 
some unknown cause, in the same 
general direction in a zigzag line ; 
but between 4.30 and 10 p.m. 
almost completed a second small 
ellipse. The cotyledons move 
only a little up and down : thus 
at 10.15 P.M. they stood only 10° 
higher than at noon. The chief 
seat of movement therefore, at 
least when the cotyledons are 
rather old as in the present case, 
lies in the hypocotyl. The ellipse 
described on the 29th had its 
longer axis directed at nearly 
right angles to a line joining 
the two cotyledons. The actual 
amount of movement of the bead 
at the end of the filament was, as far as could be ascertained, 
about '14 of an inch. 

Helianthus annuus (Compositae). — The upper part of the 
hypocotyl moved during the day-time in the course shown in 
the annexed figure (Fig. 32). As the line runs in various direc- 
tions, crossing itself several times, the movement may be con- 
sidered as one of circumnutatiou. The extreme actual distance 
travelled was at least "1 of an inch. The movements of the 
cotyledons of two seedlings were observed ; one facing a north- 
east window, and the other so feebly illuminated from above as 
to be almost in darkness. They continued to sink till about 
noon, when they began to rise; but between 5 and 7 or 8 p.m. 
they either sank a little, or moved laterally, and then again 
began to rise. At 7 a.m. on the following morning those on 



Opuntia basilaris : conjoint cir- 
cumnutation of hypocotyl and 
cotyledon ; filament fixed lon- 
gitudinally to cotyledon, and 
movement traced during 66 h. 
on horizontal glass. Move- 
ment of the terminal bead 
magnified about 30 times, 
here reduced to one - third 
scale. Seedling kept in hot- 
house, feebly illuminated from 
above. 



46 



CIRCUMNUTATION OF SEEDLINGS. Chap. I. 



X--- 



the plant before the north-east window had opened so little 
that they stood at an angle of 73° above the horizon, and were 

not observed any longer. 
Those on the seedling 
which had been kept in 
almost complete darkness, 
sank during the whole 
day, without rising about 
mid-day, but rose during 
the night. On the third 
and fourth days they con- 
tinued sinking without 
any alternate ascending 
movement; and this, no 
doubt, was due to the ab- 
sence of light. 

Primula Sinensis (Pri- 
mulaceee). — A seedling was 
placed with the two cotyl- 
edons parallel to a north-east window on a day when the light 
was nearly uniform, and a filament was affixed to one of them. 
From observations subsequently made on another seedling with 
the stem secured to a stick, the greater part of the movement 
shown in the annexed figure (Fig. 33), must have been that of 




Helianthus annuus: circumnutation of 
hypocotyl, with filament fixed across 
its sunmiit, traced on a horizontal 
glass in darkness, from 8.45 a.m. to 
10.45 P.M., and for an hour on follow- 
ing morning. Movement of bead 
magnified 21 times, here reduced to 
one-half of original scale. 




Primula Sinensis : conjoint circumnutation of hypocotyl and cotyledon, 
traced on vertical glass, from 8.40 a.m. to 10.45 p.m. Movements 
of bead magnified about 26 times. 



the hypocotyl, though the cotyledons certainly move up and 
down to a certain extent both during the day and night. The 
movements of the same seedling were traced on the followino; 



Chap. I. IPOMCEA. 47 

day with nearly the same result; and there can be no doubt 
about the circumnutation of the hypocotyl. 

Cyclamen Persicum (Primulacese). — This plant is generally 
supposed to produce only a single cotyledon, but Dr. H. Gress- 
ner*. has shown that a second one is developed after a long 
interval of time. The hypocotyl is converted into a globular 
corm, even before the first cotyledon has broken through the 
ground with its blade closely enfolded and with its petiole in 
the form of an arch, like the arched hypocotyl or epicotyl of 
any ordinary dicotyledonous plant. A glass filament was af- 
fixed to a cotyledon, -55 of an inch in height, the petiole of 
which had straightened itself and stood nearly vertical, but 
with the blade not as yet fully expanded. Its movements were 
traced during 24:^ h. on a horizontal glass, magnified 50 times; 
and in this interval it described 
two irregular small circles; it ^ ■^^^' ^^* 

therefore circumnutates, though \ ^ — *.. 

on an extremely small scale. \ r-^.^^ V ^^ 

Stapelia sarpedon (Asclepia- V ^s" ^ ) 

deaej.-This plant, when ma- stapelia sarpedon : <,ircx^mmxt^tion 
ture, resembles a cactus. The of hypocotyl, illuminated from 
flattened hypocotyl is fleshy, en- l^-^V.^el a°.°h. 5^"^' 
larged in the upper part, and to 8.45 a.m. 28th. Temp. 23°- 
bears two rudimentary cotyle- ^4° a ^Mo^ement of bead mag- 
dons. It breaks through the 

ground in an arched form, with the rudimentary cotyledons 
closed or in contact. A filament was affixed almost vertically 
to the hypocotyl of a seedling half an inch high; and its 
movements were traced during 50 h. on a horizontal glass 
(Fig. 34). From some unknown cause it bowed itself to one 
side, and as this was effected by a zigzag course, it probably 
circumnutated ; but with hardly any other seedling observed 
by us was this movement so obscurely shown. 

Ipom(Ea ccerulea vel PharUtis nil (Convolvulacese). — Seed- 
lings of this plant were observed because it is a twiner, the 
upper internodes of which circumnutate conspicuously; but, 
like other twining plants, the first few internodes which rise 



* 'Bot. Zeitung,' 1874, p. 837. 



48 CIRCUMNUTATION OF SEEDLINGS. Chap. I. 

above the ground are stiff enough to support themselves, and 
therefore do not circumnutate in any plainly recognisable man- 
ner.* In this particular instance the fifth internode (including 
the hypocotyl) was the first which plainly circumnutated and 
twined round a stick. We therefore wished to learn whether 
circumnutation could be observed in the hypocotyl if carefully 
observed in our usual manner. Two seedlings were kept in the 
dark with filaments fixed to the upper part of their hypocotyls ; 
but from circumstances not worth explaining their movements 
were traced for only a short time. One moved thrice forwards 
and twice backwards in nearly opposite directions, in the course 
of 3 h. 15 ra. ; and the other twice forwards and twice back- 
wards in 2 h. 22 m. The hypocotyl therefore circumnutated at 
a remarkably rapid rate. It may here be added that a filament 
was aflaxed transversely to the summit of the second internode 
above the cotyledons of a little plant S^ inches in height ; and 
its movements were traced on a horizontal glass. It circum- 
nutated, and the actual distance travelled from side to side was 
a quarter of an inch, which w^as too small an amount to be per- 
ceived without the aid of marks. 

The movements of the cotyledons are interesting from their 
complexity and rapidity, and in some other respects. The 
hypocotyl (2 inches high) of a vigorous seedling was secured to 
a stick, and a filament with triangles of paper v/as affixed to one 
of the cotyledons. The plant was kept all day in the hot-house, 
and at 4.20 p.m. (June 20th) was placed under a skylight in 
the house, and observed occasionally during the evening and 
night. It fell in a slightly zigzag line to a moderate extent 
from 4.20 p.m. till 10.15 p.m. When looked at shortly after 
midnight (12.30 p.m.) it had risen a very little, and consider- 
ably by 3.45 A.M. When again looked at, at 6.10 a.m. (21st), 
it had fallen largely. A new tracing was now begun (see Fig. 
35), and soon afterwards, at 6.42 a.m., the cotyledon had risen 
a little. During the forenoon it was observed about every hour; 
but between 12.30 and 6 p.m. every half-hour. If the observa- 
tions had been made at these short intervals during the whole 
day, the figure would have been too intricate to have been 



* 'Movements and Habits of Climbing Plants,' p. 33, 1875. 



Chap. I. 



IPOMCEA. 



49 



copied. As it was, the cotyl- 
edon moved up and down 
in the course of 16 h. 20 m. 
(i.e. between 6.10 a.m. and 
10.30 P.M.) thirteen times. 

The cotyledons of this 
seedling sank downwards 
during both evenings and the 
early part of the night, but 
rose during the latter part. 
As this is an unusual move- 
ment, the cotyledons of 
twelve other seedlings were 
observed; they stood almost 
or quite horizontally at mid- 
day, and at 10 p.m. were all 
declined at various angles. 
The most usual angle was be- 
tween 30° and 35° ; but three 
stood at about 50° and one at 
even 70° beneath the horizon. 
The blades of all these cotyl- 
edons had attained almost 
their full size, viz. from 1 to 
1| inches in length, meas- 
ured along their midribs. It 
is a remarkable fact that 
whilst young — that is, when 
less than half an inch in 
length, measured in the same 
manner — they do not sink 
downwards in the evening. 
Therefore their weight, which 
is considerable when almost 
fully developed, probably 
came into play in originally 
determining the downward 
movement. The periodicity 
of this movement is much in- 



Fig. 35. 




6\lOM.m 2J'J 



Ipomdp.a cxrulea : circumnutation of 
cotyledon, traced on vertical glass, 
from 6.10 A.M. June 21st to 6.45 
A.M. 22nd. Cotyledon with petiole 
1'6 inch in length, apex of blade 
4"1 inch from the vertical glass ; so 
movement not greatly magnified ; 
temp. 20° C. 



50 CmCUMNUTATION OF SEEDLINGS. Chap. I. 

fluenced by the degree of light to which the seedlings have 
been exposed during the day; for three kept in an obscure 
place began to sink about noon, instead of late in the evening; 
and those of another seedling were almost paralysed by having 
been similarly kept during two whole days. The cotyledons 
of several other species of Ipomoea likewise sink downwards 
late in the evening. 

Cerinthe major (BoragineoB). — The circumnutation of the 
hypocotyl of a young seedling with the cotyledons hardly ex- 
panded, is shown in the annexed figure (Fig. 36), which appar- 

Fig. 36. 




Cerinthe major : circumnutation of hypocotyl, with filament fixed across 
its summit, illuminated from above, traced on horizontal glass, from 
9.26 A.M. to 9.53 P.M. on Oct. 25th. Movement of the bead magni- 
fied 30 times, here reduced to one-third of original scale. 

ently represents four or five irregular ellipses, described in the 
course of a little over 12 hours. Two older seedlings were 
similarly observed, excepting that one of them was kept in the 
dark ; their hypocotyls also circumnutated, but in a more simple 
manner. The cotj^ledons on a seedling exposed to the light 
fell from the early morning until a little after noon, and then 
continued to rise until 10.30 p.m. or later. The cotyledons of 
this same seedling acted in the same general manner during the 
two following days. It had previously been tried in the dark, 
and after being thus kept for only 1 h. 40 m. the cotyledons 
began at 4.30 p.m. to sink, instead of continuing to rise till late 
at night. 

Nolana prostrata (JSTolanefe). — The movements were not 
traced, but a pot with seedlings, which had been kept in the 



Chap. I. SOLANUM. 51 

dark for an hour, was placed under the microscope, with the 
micrometer eye-piece so adjusted that each division equalled 
^th of an inch. The apex of one of the cotyledons crossed 
rather obliquely four divisions in 13 minutes; it was also sink- 
ing, as shown by getting out of focus. The seedlings were 
again placed in darkness for another hour, and the apex now 
crossed two divisions in 6 m. 18 s. ; that is, at very nearly the 
same rate as before. After another interval of an hour in dark- 
ness, it crossed two divisions in 4 m. 15 s., therefore at a quicker 
rate. In the afternoon after a longer interval in the dark, the 
apex was motionless, but after a time it recommenced moving, 
though slowly ; perhaps the room was too cold. Judging from 
previous cases, there can hardly be a doubt 
that this seedling was circumnutating. ^" 

Solanum lycopersicum (Solanese). — The move- 
ments of the hypocotyls of two seedling toma- 
toes were observed during seven hours, and 
there could be no doubt that both circumnu- 
tated. They were illuminated from above, 
but by an accident a little light entered on one 
side, and in the accompanying figure (Fig. 37) 
it may be seen that the hypocotyl moved to g^^^^^ ly^^^^^. 
this side (the upper one in the figure), making sicum: circum- 
small loops and zigzagging in its course. The "^llfj^ ""^vmx 
movements of the cotyledons were also traced filament fixed 
both on vertical and horizontal glasses; their mH^ to^ced"^^ 
angles with the horizon were likewise meas- horizontal glass, 
ured at various hours. They fell from 8.30 s^'p^. oct'sith" 
A.M. (October 17th) to about noon; then moved Illuminated ob- 
laterally in a zigzag line, and at about 4 p.m. above^ MovS 
began to rise ; they continued to do so until ment of bead 
10. 30 P.M. , by which hour they stood vertically ^g^'^Smg^ ^here 
and were asleep. At what hour of the night reduced to one- 
or early morning they began to fall was not g^ale ^^^^^^^"^^^ 
ascertained. Owing to the lateral movement 
shortly after mid-day, the descending and ascending lines did 
not coincide, and irregular ellipses were described during each 
24 h. The regular periodicity of these movements is destroyed, 
as we shall hereafter see, if the seedlings are kept in the dark. 




52 CIRCUMNUTATION OF SEEDLINGS. Chap. L 

Solanum palinacanthum. — Several arched hypocotyls rising 
nearly -2 of an inch above the ground, but with the cotyledons 
still buried beneath the surface, were observed, and the tracings 

Fig. 38. 




Solanum palinacanthum : circumnutation of an arched hypocotyl, just 
emerging from the ground, with the two legs tied together, traced 
in darkness on a horizontal glass, from 9.20 a.m. Dec. 17th to 8.30 
A.M. 19th. Movement of bead magnified 13 times ; but the fila- 
ment, which was affixed obliquely to the crown of the arch, was of 
unusual length. 

showed that they circumnutated. Moreover, in several cases 
little open circular spaces or cracks in the argillaceous sand 
which surrounded the arched hypocotyls were visible, and these 
appeared to have been made by the hypocotyls having bent first 
to one and then to another side whilst growing upwards. In 
two instances the vertical arches were observed to move to 
a considerable distance backwards from the point where the 
cotyledons lay buried ; this movement, which has been noticed 
in some other cases, and which seems to aid in extracting the 
cotyledons from the buried seed-coats, is due to the commence- 
ment of the straightening of the hypocotyl. In order to pre- 
vent this latter movement, the two legs of an arch, the summit 
of which was on a level with the surface of the soil, were tied 
together; the earth having been previously removed to a little 
depth all round. The movement of the arch during 47 hours 
under these unnatural circumstances is exhibited in the annexed 
figure. 

The cotyledons of some seedlings in the hot-house were 
horizontal about noon on December 13th ; and at 10 p.m. had 



Chap. I. 



BETA. 



53 



Fig. 39. 



risen to an angle of 37° above the horizon; at 7 a.m. on the 
following morning, before it was light, they had risen to 59° 
above the horizon; in the afternoon of the same day they were 
found again horizontal. 

Beta vulgaris (Chenopodese). — The seedlings are excessively 
sensitive to light, so that altliough on the first day they were 
uncovered only during two or three minutes at each observation, 
they all moved steadily towards the side of the room whence 
the light proceeded, and the tracings consisted only of slightly 
zigzag lines directed towards the light. On the next day the 
plants were placed in a completely darkened room, and at each 
observation were illuminated as much as possible from vertically 
above by a small wax taper. The annexed figure (Fig. 39) 
shows the movement of the hypocotyl 
during 9 h. under these circumstances. 
A second seedling was similarly observed 
at the same time, and the tracing had 
the same peculiar character, due to the 
,hypocotyl often moving and returning 
in nearly parallel lines. The movement 
of a third hypocotyl differed greatly. 

We endeavoured to trace the move- 
ments of the cotyledons, and for this 
purpose some seedlings were kept in the 
dark, but they moved in an abnormal 
manner ; they continued rising from 8.45 
A.M. to 2 P.M., then moved laterally, and 
from 3 to 6 p.m. descended; whereas 
cotyledons which have been exposed all 
the day to the light rise in the evening 
so as to stand vertically at night; but 
this statement applies only to young 
seedlings. For instance, six seedlings 
in the greenhouse had their cotyledons 
partially open for the first time on the morning of November 
15th, and at 8.45 p.m. all were completely closed, so that they 
might properly be said to be asleep. Again, on the morning 
of November 27th, the cotyledons of four other seedlings 
which were surrounded by a collar of brown paper so that they 




Beta vulgaris : circumuu- 
tation of hypocotyl, 
with filament fixed ob- 
liquely across its sum- 
mit, traced in darkness 
on horizontal glass, 
from 8.25 A.M. to 5.30 
P.M. Nov. 4th. Move- 
ment of bead magni- 
fied 23 times, here re- 
duced to one-third of 
original scale. 



54 CmCUMNUTATlON OF SEEDLINGS. Chap. I. 

received light only from above, were open to the extent of 
39°; at 10 p.m. they were completely closed; next morning 
(November 28th) at 6.45 a.m., whilst it was still dark, two of 
them were partially open and all opened in the course of the 
morning; but at 10.30 p.m. all four (not to mention nine others 
which had been open in the morning and six others on another 
occasion) were again completely closed. On the morning of the 
29th they were open, but at night only one of the four was 
closed, and this only partially; the three others had their 
cotyledons much more raised than during the day. On the 
night of the 30th the cotyledons of the four were only slightly 
raised. 

Ricinus Borboniensis (Euphorbiaceae) . — Seeds were purchased 
under the above name — probably a variety of the common castor- 
oil plant. As soon as an arched hypocotyl had risen clear above 
the ground, a filament was attached to the upper leg bearing the 
cotyledons which were still buried beneath the surface, and the 
movement of the bead was traced on a horizontal glass during 
a period of 34 h. The lines traced were strongly zigzag, and 
as the bead twice returned nearly parallel to its former course 
in two different directions, there could be no doubt that the 
arched hypocotyl circumnutated. At the close of the 34 h. 
the upper part began to rise and straighten itself, dragging the 
cotyledons out of the ground, so that the movements of the 
bead could no longer be traced on the glass. 

Quercus (American sp. ) (Cupuliferse). — Acorns of an Ameri- 
can oak which had germinated at Kew were planted in a pot 
in the greenhouse. This transplantation checked their growth ; 
but after a time one grew to the height of five inches, 
measured to the tips of the small partially unfolded leaves on 
the summit, and now looked vigorous. It consisted of six 
very thin internodes of unequal lengths. Considering these 
circumstances and the nature of the plant, we hardly expected 
that it would circumnutate ; but the annexed figure (Fig. 40) 
shows that it did so in a conspicuous manner, changing its 
course many times and travelling in all directions during the 
48 h. of observation. The figure seems to represent 5 or 6 
irregular ovals or ellipses. The actual amount of movement 
from side to side (excluding one great bend to the left) was 



Chap. I. 



QUERCUS. 



55 



about 2 of an inch ; but this was diflScult to estimate, as owing 
to the rapid growth of the stem, the attached filament was 
much further from the mark beneath at the close than at the 




Quercus (American sp.) : circumnutation of young stem, traced on 
horizontal glass, from 12.50 p.m. Feb. 22nd to 12.50 p.m. 24th. 
Movement of bead greatly magnified at first, but slightly towards 
the close of the observations — about 10 times on an average. 

commencement of the observations. It deserves notice that the 
pot was placed in a north-east room within a deep box, the top 
of which was not at first covered up, so that tlie inside facing 
the windows was a little more illuminated than the opposite 
side ; and during the first morning the stem travelled to a 
greater distance in this direction (to the left in the figure) than 
it did afterwards when the box was completely protected from 
light. 

Quercus rolur. — Observations were made only on the move- 
ments of the radicles from germinating acorns, which were allowed 
to grow downwards in the manner previously described, over 
plates of smoked glass, inclined at angles between 65 ° and 69 ° 
to the horizon. In four cases the tracks left were almost straight, 



56 CIRCUMNUTATION OF SEEDLINGS. Chap. I. 

but the tips had pressed sometimes with more and sometimes 
with less force on the glass, as shown by the varying thickness 
of the tracks and by little bridges of soot left across them. 
In the fifth case the track was slightly serpentine, that is, the 
tip had moved a little from side to side. In the sixth case 
(Fig. 41, A) it was plainly serpentine, and the tip had pressed 
almost equably on the glass in its whole course. In the seventh 
case (B) the tip had moved both laterally and had pressed 
alternately with unequal force on the glass; so that it had 
moved a little in two planes at right angles to one another. In 
the eighth and last case (C) it had moved very little laterally, 
but had alternately left the glass and came into contact with it 

Fisf. 41. 





Quercus rohur : tracks left on inclined smoked glass-plates by tips of 
radicles in growing downwards. Plates A and C inclined at 65° 
and plate B at 68° to the horizon. 

again. There can be no doubt that in the last four cases the 
radicle of the oak circummitated whilst growing downwards. 

Corylus amllana (Corylaceae). — The epicotyl breaks through 
the ground in an arched form ; but in the specimen which was 
first examined, the apex had become decayed, and the epicotyl 
grew to some distance through the soil, in a tortuous, almost 
horizontal direction, like a root. In consequence of this injury 
it had emitted near the hypogean cotyledons two secondary 
shoots, and it was remarkable that both of these were arched, 
like the normal epicotyl in ordinary cases. The soil was removed 



Chap. I. 



PINUS. 



57 



leg. The whole was kept 
Fig. 42. 



from around one of these arched secondary shoots, and a glass 
filament was affixed to the basal 
damp beneath a metal- box with a 
glass lid, and was thus illuminated 
only from above. Owing apparent- 
ly to the lateral pressure of the 
earth being removed, the terminal 
and bowed-down part of the shoot 
began at once to move upwards, 
so that after 24 h. it formed a 
right angle with the lower part. 
This lower part, to which the fila- 



\ 




Corylus avellana : circumnuta- 
tion of a young shoot emitted 
from the epicotyl, the apex 
of which had been injured, 
traced on a horizontal glass, 
from 9 A.M. Feb. 2nd to 8 
A.M. 4th. Movement of bead 
magnified about 27 times. 



ened itself, and moved a little 
backwards from the upper part. 
Consequently a long line was 
traced on the horizontal glass ; 
and this was in parts straight and 
in parts decidedly zigzag, indicat- 
ing . circumnutation. 

On the following day the other secondary shoot was observed ; 
it was a little more advanced in age, for the upper part, instead 
of depending vertically downwards, stood at an angle of 45° above 
the horizon. The tip of the shoot projected obliquely -4 of an 
inch above the ground, but by the close of our observations, 
which lasted 47 h., it had grown, chiefly towards its base, to a 
height of -85 of an inch. The filament was fixed transversely 
to the basal and almost upright half of the shoot, close beneath 
the lowest scale-like appendage. The circumnutating course 
pursued is shown in the accompanying figure (Fig. 42). The 
actual distance traversed from side to side was about '04 of an 
inch. 

Pinus pinaster (Coniferse).— A young hypocotyl, with the 
tips of the cotyledons still enclosed Avithin the seed-coats, was 
at first only '35 of an inch in height ; but the upper part grew 
so rapidly that at the end of our observations it was -6 in height, 
and by this time the filament was attached some way down the 
little stem. From some unknown cause, the hypocotyl moved 
far towards the left, but there could be no doubt (Fig. 43) that 



68 



CIRCUMNUTATION OF SEEDLINGS. Chap. I. 



it circumnutated. Another hypocotyl was similarly observed, 
and it likewise moved in a strongly zigzag line to the same side. 



Fig. 43, 




Pinus pinaster : circumnutation of hypocotyl, with filament fixed across 
its summit, traced on horizontal glass, from 10 a.m. March 21st to 
9 A.M. 23rd. Seedling kept in darkness. Movement of bead mag- 
nified about 35 times. 

This lateral movement was not caused by the attachment of 
the glass filaments, nor by the action of light ; for no light was 
allowed to enter when each observation was made, except from 
vertically above. 

The hypocotyl of a seedling was secured to a little stick ; it 
bore nine in appearance distinct cotyledons, arranged in a circle. 
The movements of two nearly opposite ones were observed. 

Fig. 44. 




/ 



A. 




Pinus pinaster : circumnutation of two opposite cotyledons, traced on 
horizontal glass in darkness, from 8.45 a.m. to 8.35 p.m. Nov. 25th. 
Movement of tip in A magnified about 22 times, here reduced to 
one-half of original scale. 

The tip of one was painted white, with a mark placed below, 
and the figure described (Fig. 44, A) shows that it made an 



Chap. I. 



CYGAS. 



69 



irregular circle in the course of about 8 h. During the night it 
travelled to a considerable distance in the direction indicated 
by the broken line. A glass filament was attached longitudinally 
to the other cotyledon, and this nearly completed (Fig. 44, B) 
an irregular circular figure in about 13 hours. During the 
night it also moved to a considerable distance, in the direction 
indicated by the broken line. The cotyledons therefore circum- 
nutate independently of the movement of the hypocotyl. 
Although they moved much during the night, they did not 
approach each other so as to stand more vertically than during 
the day. 

Cycas pectiTiata (Cycadeas). — The large seeds of this plant in 
germinating first protrude a single leaf, which breaks through 




Cycas pectinata : circumnutation of young leaf whilst emerging from 
the ground, feebly illuminated from above, traced on vertical 
glass, from 5 p.m. May 28th to 11 A.M. 31st. Movement magnified 
7 times, here reduced to two-thirds of original scale. 

the ground with the petiole bowed into an arch and with the 
leaflets involuted. A leaf in this condition, which at the close 
of our observations was 2^ inches in height, had its movements 
traced in a warm greenhouse by means of a glass filament 
bearing paper triangles attached across its tip. The tracing 
(Fig. 45) shows how large, complex, and rapid w^ere the circum- 



GO 



CIRCUMNUTATION OF SEEDLINGS. Chap. I. 



nutating movements. The extreme distance from side to side 
which it passed over amounted to between -6 and -7 of an inch. 
Carina Warsceiciczii (Cannaceae). — A seedling with the plu- 
mule projecting one inch above the ground was observed, but 
not under fair conditions, as it was brought out of tlie hot- 
house and kept in a room not suf- 
ficiently warm. Nevertheless the 
tracing (Fig. 46) shows that it 
made two or three incomplete ir- 
regular circles or ellipses in the 
course of 48 hours. The plumule 
is straight ; and this was the first 
instance observed by us of the 
part that first breaks through the 
ground not being arched. 

Allium cejpa (Liliacete). — The 
narrow green leaf, which protrudes 
from the seed of the common onion 
as a cotyledon, * breaks through the 
ground in the form of an arch, in 
the same manner as the hypocotyl 
or epicotyl of a dicotyledonous 
plant. Long after the arch has 
risen above the surface the apex 
remains within the seed-coats, evi- 
dently absorbing the still abun- 
dant contents. The summit or crown of the arch, when it first 
protrudes from the seed and is still buried beneath the ground, 
is simply rounded ; but before it reaches the surface it is devel- 
oped into a conical protuberance of a white colour (owing to the 
absence of chlorophyll), whilst the adjoining parts are green, with 
the epidermis apparently rather thicker and tougher than else- 
where. We may therefore conclude that this conical protuber- 
ance is a special adaptation for breaking througli the ground,! 




Canna Warscewksii : circum- 
nutation of plumule with 
filament affixed obliquely to 
outer slieath-like leaf, traced 
in darkness on horizontal 
glass from 8.45 a.m. Nov. 
9th to 8.10 A.M. nth. Move- 
ment of bead magnified 6 
times. 



* This is the expression used by 
Sachs in his ' Text book of Botany. ' 

t Haberlandt has briefly de- 
scribed ('Die Schutzeinrichtun- 
gen. . . . Kcimpflanzc,' 1877, p. 
77) this curious structure and tlie 



purpose wbich it subserves. He 
states tliat good figures of the 
cotyledon of the onion have been 
given by Tittmann and by Sachs 
in bis ' Experimental Physiologic,' 
p. 93. 



Chap. I. 



ASPARAGUS. 



61 



and answers the same end as the knife-like white crest on 
the summit of the straight cotyledon of the Graminese. After 
a time the apex is drawn out of the empty seed-coats, and rises 
up, forming a right angle, or more commonly a still larger angle 
with the lower part, and occasionally the whole becomes nearly 
straight. The conical protuberance, which originally formed 
the crown of the arch, is now seated on one side, and appears 
like a joint or knee, which from acquiring chlorophyll becomes 
green, and increases in size. In rarely or never becoming per- 
fectly straight, these cotyledons differ remarkably from the 
ultimate condition of the arched hypocotyls or epicotyls of 
dicotyledons. It is, also, a singular circumstance that the 
attenuated extremity of the upper bent portion invariably 
withers and dies. 

A filament, 1*7 inch in length, was 'affixed nearly upright 
beneath the knee to the basal and vertical portion of a cotyledon ; 
and its movements were traced during 14 h. in the usual manner. 
The tracing here given (Fig. 47) indicates circumnutation. 
The movement of the upper part above 
the knee of the same cotyledon, which 
projected at about an angle of 45° above 
the horizon, was observed at the same 
time. A filament was not affixed to it, 
but a mark was placed beneath the 
apex, which was almost white from be- 
ginning to wither, and its movements 
were thus traced. The figure described 
resembled pretty closely that above 
given; and this shows that the chief 
seat of movement is in the lower or basal 
part of the cotyledon. 

Asparagus officinalis (Asparagse). — 
The tip of a straight plumule or cotyle- 
don (for we do not know which it should 
be called) was found at a depth of -1 
inch beneath the surface, and the earth 

was then removed all round to the depth of -3 inch. A glass 
filament was affixed obliquely to it, and the movement of the 
bead, magnified 17 times, was traced in darkness. During the 



Fig. 47. 




Allinm cepa : circumnu- 
tation of basal half 
of arched cotyledon, 
traced iu darkness on 
horizontal glass, from 
8.15 A.M. to 10 P.M. 
Oct. 31st. Movement 
of bead magnified 
about 17 times. 



62 CIRCUMNUTATION OF SEEDLINGS. Chap. I. 

first 1 h. 15 m. the plumule moved to the right, and during the 
next two hours it returned in a roughly parallel but strongly- 
zigzag course. From some unknown cause it had grown up 
through the soil in an inclined direction, and now through 
apogeotropism it moved during nearly 24 h. in the same gen- 
eral direction, but in a slightly zigzag manner until it became 
upright. On the following morning it changed its course com- 
pletely. There can therefore hardly be a doubt that the 
plumule circumnutates, whilst buried beneath the ground, as 
much as the pressure of the surrounding earth will permit. 
The surface of the soil in the pot was now covered with a thin 
layer of very fine argillaceous sand, which was kept damp ; and 
after the tapering seedlings had grown a few tenths of an inch 
in height, each was found surrounded by a little open space or 
circular crack ; and this could have been accounted for only by 
their having circumnutated and thus pushed away the sand on 
all sides ; for there was no vestige of a crack in any other part. 
In order to prove that there was circumnutation, the move- 
ments of five seedlings, varying in height from '3 inch to 2 
inches, were traced. They were placed within a box and illu- 
minated from above ; but in all five cases the longer axes of the 
figures described were directed to nearly the same point; so 
that more light seemed to have come through the glass roof of 
the greenhouse on one side than on any other. All five tracings 
resembled each other to a certain extent, and it will sufiice 
to give two of them. In A (Fig. 48) the seedling was only 
•45 of an inch in height, and consisted of a single internode 
bearing a bud on its summit. The apex described between 
8.30 A.M. and 10.20 p.m. (i.e. during nearly 14 hours) a figure 
which would probably have consisted of 3^ ellipses, had not 
the stem been drawn to one side until 1 p.m. after which hour 
it moved backwards. On the following morning it w^as not far 
distant from the point whence it had first started. The actual 
amount of movement of the apex from side to side was very 
small, viz. about yV^h of an inch. The seedling of which the 
movements are shown in Fig. 48, B, was If inch in height, and 
consisted of three intern odes besides the bud on the summit. 
The figure, which was described during 10 h., apparently rep- 
resents two irregular and unequal ellipses or circles. The actual 



Chap. I. 



PHALARIS. 



63 



amount of movement of the apex, in the line not influenced by 
the light, was -11 of an inch, and in that thus influenced -37 of 
an inch. With a seedling 2 inches in height it was obvious, 



Fig. 48. 





A. B. 

Asparagus officinalis : circumnutation of plumules with tips whitened 
and marks placed beneath, traced on a horizontal glass. A, young 
plumule ; movement traced from 8.30 a.m. ISTov. 30th to 7.15 a.m. 
next morning ; magnified about 35 times. B, older plumule ; move- 
-. ment traced from 10.15 A.M. to 8.10 p.m. Nov. 29th ; magnified 
9 times, but here reduced to one-half of original scale. 

even without the aid of any tracing, that the uppermost part of 
the stem bent successively to all points of the compass, like the 
stem of a twining plant. A little increase in the power of 
circumnutating and in the flexibility of the stem, would convert 
the common asparagus into a twining plant, as has occurred 
with one species in this genus, namely, A. scandens. 

Phalaris Canariensis (Gramineae). — With the Grramineae the 
part which first rises above the ground has been called by some 
authors the pileole ; and various views have been expressed on 
its homological nature. It is considered by some great authori- 
ties to be a cotyledon, which term we will use without venturing 
to express any opinion on the subject.* It consists in the 
present case of a slightly flattened reddish sheath, terminating 



"••■ We are indebted to the Eev. 
G. Henslow for an abstract of the 
views which have been held on 



this subject, together with refer- 
ences. 



64 



CmCUMNUTATION OF SEEDLINGS. Chap. I. 



Fig. 49. 




upwards in a sharp white edge ; it encloses a true green leaf, 
which protrudes from the sheath through a slit-like orifice, 

close beneath and at right angles 
to the sharp edge on the summit. 
The sheath is not arched when it 
breaks through the ground. 

The movements of three rather 
old seedlings, about 1^ inch in 
height, shortly before the protru- 
sion of the leaves, were first traced. 
They were illuminated exclusive- 
ly from above ; for, as will here- 
after be shown, they are exces- 
sively sensitive to the action of 
light ; and if any enters even tem- 
porarily on one side, they merely 
bend to this side in slightly zig- 
zag lines. Of the three tracings 
one alone (Fig. 49) is here given. Had the observations been 
more frequent during the 12 h. two oval figures would have 
been described with their longer axes at right angles to one 
another. The actual amount of movement of the apex from 
side to side was about -3 of an inch. The figm-es described by 
the other two seedlings resembled 
to a certain extent the one here 



Phalaris Canariensis : circumnu- 
tation of a cotyledon, with a 
mark placed below the apex, 
traced on a horizontal glass, 
from 8.35 A.M. Nov. 26th to 
8.45 A.M. 27th. Movement of 
apex magnified 7 times, here 
reduced to one-half scale. 



Fig. 50, 




given. 

A seedling which had just 
broken through the ground and 
projected only ^^ of an inch 
above the surface, was next ob- 
served in the same manner as be- 
fore. It was necessary to clear 
away the earth all round the 
seedling to a little depth in order 
to place a mark beneath the apex. 
The figure (Fig. 50) shows that 
the apex moved to one side, but 
changed its course ten times in the course of the ten hours of 
observation ; so that there can be no doubt about its circum- 



Phalaris Canariensis : circumnu- 
tation of a very young coty- 
ledon, with a mark placed be- 
low the apex, traced on a 
horizontal glass, from 11.37 
A.M. to 9.30 P.M. Dec. 13th. 
Movement of apex greatly 
magnified, here reduced to 
one-fourth of original scale. 



Chap. I. PHALARIS. 65 

nutation. The cause of the general movement in one direction 
could hardly be attributed to the entrance of lateral light, as 
this was carefully guarded against ; and we suppose it was in 
some manner connected with the removal of the earth round 
the little seedling. 

Lastly, the soil in the same pot was searched with the aid 
of a lens, and the white knife-like apex of a seedling was found 
on an exact level with that of the surrounding surface. The 
soil was removed all round the apex to the depth of a quarter 
of an inch, the seed itself remaining covered. The pot, pro- 
tected from lateral light, was placed under the microscope with 
a micrometer eye-piece, so arranged that each division equalled 
■^ of an inch. After an interval of 30 m. the apex was ob- 
served, and it was seen to cross a little obliquely two divisions 
of the micrometer in 9 m. 15 s. ; and after a few minutes it 
crossed the same space in 8 m. 50 s. The seedling was again 
observed after an interval of three-quarters of an hour, and now 
the apex crossed rather obliquely two divisions in 10 m. We 
may therefore conclude that it was travelling at about the rate 
of -^^th of an inch in 45 minutes. We may also conclude from 
these and the previous observations, that the seedlings of Pha- 
laris in breaking through the surface of the soil circumnutate 
as much as the surrounding pressure will permit. This fact 
accounts (as in the case before given of the asparagus) for a 
circular, narrow, open space or crack being distinctly visible 
round several seedlings which had risen through very fine 
agrillaceous sand, kept uniformly damp. 

Zea mays (Graminese). — A glass filament was fixed obliquely 
to the summit of a cotyledon, rising '2 of an inch above the 
ground; but by the third morning it had grown to exactly 
thrice this height, so that the distance of the bead from the 
mark below was greatly increased, consequently the tracing 
(Fig. 51) was much more magnified on the first than on the 
second day. The upper part of the cotyledon changed its 
course by at least as much as a rectangle six times on each of 
the two days. The plant was illuminated by an obscure light 
from vertically above. This was a necessary precaution, as on 
the previous day we had traced the movements of cotyledons 
placed in a deep box, the inner side of which was feebly illu- 



g; 



CmCUMNUTATION OF SEEDLINGS. Chap. I 




Zea mays : circumnutatiou of cotyle- 
don, traced on horizontal glass, 
from 8.30 A.M. Feb. 4.h to 8 A.M. 
6th. Movement of bead magni- 
fied on an average about 25 times. 



minated on one side from a distant north-east window, and at 
each observation by a wax taper held for a minute or two on the 
same side ; and the result was that the cotyledons travelled all 
day long^ to this side, though making in their course some con- 
spicuous flexures, from which 
fact alone we might have 
concluded that they were cir- 
cumnutating ; but we thought 
it advisable to make the tra- 
cing above given. 

Radicles. — Glass filaments 
were fixed to two short radi- 
cles, placed so as to stand 
almost upright, and whilst 
bending downwards through 
geotropism their courses were 
strongly zigzag; from this 
latter circumstance circum- 
nutatiou might have been in- 
ferred, had not their tips be- 
come slightly withered after the first 24 h., though they were 
watered and the air kept very damp. Nine radicles were next 
arranged in the manner formerly described, so that in growing 
downwards they left tracks on smoked glass- 
plates, inclined at various angles between 45° 
and 80° beneath the horizon. Almost every 
one of these tracks offered evidence in their 
greater or less breadth in different parts, or 
in little bridges of soot being left, that the 
apex had come alternately into more and less 
close contact with the glass. In the accom- 
panying figure (Fig. 52) we have an accurate 
copy of one such track. In two instances 
alone (and in these the plates were highly 
inclined) there was some evidence of slight 
lateral movement. We presume therefore that 
the friction of the apex on the smoked surface, 
little as this could have been, sufficed to check the movement 
from side to side of these delicate radicles. 



Fig. 52. 



Zea mays: track 
left on inclined 
smoked glass- 
plate by tip of 
radicle in grow- 
ing downwards. 



Chap. I. 



NEPHRODIUM. 



67 



Avena sativa (Graminese). — A cotyledon, 1^ inch in height, 
was placed in front of a north-east window, and the movement 
of the apex was traced on a horizontal glass during two days. 
It moved towards the light in a slightly zigzag line from 9 to 
11.30 A.M. on October 15th; it then moved a little backwards 
and zigzagged much until 5 p.m., after which hour, and during 
the night, it continued to move towards the window. On the 
following morning the same movement was continued in a 
nearly straight line until 12.40 p.m., when the sky remained 
until 2.35 extraordinarily dark from thunder-clouds. During 
this interval of 1 h. 55 m., whilst the light was obscure, it was 
interesting to observe how circumnutation overcame heliotro- 
pism, for the apex, instead of continuing to move towards the 
window in a slightly zigzag line, reversed its course four times, 
making two small narrow ellipses. A diagram of this case 
will be given in the chapter on Heliotropism. 

A filament was next fixed to a cotyledon only i of an inch 
in height, which was illuminated exclusively from above, and as 
it was kept in a warm greenhouse, it grew rapidly ; and now 
there could be no doubt about 
its circumnutation, for it de- 
scribed a figure of 8 as well as 
two small ellipses in 5^ hours. 

NepTirodium molle (Filices) . 
— A seedling fern of this spe- 
cies came up by chance in a 
flower - pot near its parent. 
The frond, as yet only slightly 
lobed, was only '16 of an inch 
in length and "2 in breadth, 
and was supported on a rachis 
as fine as a hair and '23 of an 
inch in height. A very thin 
glass filament, which projected 
for a length of '36 of an inch, was fixed to the end of the frond. 
The movement was so highly magnified that the figure (Fig. 
53) cannot be fully trusted ; but the frond was constantly mov- 
ing in a complex manner, and the bead greatly changed its 
course eighteen times in the 12 hours of observation. "Within 




Nephrodium molle : circumnutation 
of very young frond, traced iu 
darkness on horizontal glass, 
from 9 A.M. to 9 p.m. Oct. 30th. 
Movement of bead magnified 48 
times. 



68 



CIRCUMNUTATION OF SEEDLINGS. Chap. I. 



half an hour it often returned in a line almost parallel to its 
former course. The greatest amount of movement occurred 
between 4 and 6 p.m. The circumnutation of this plant is in- 
teresting, because the species in the 
genus Lygodium are well known to 
circumnutate conspicuously and to 
twine round any neighbouring ob- 
ject. 

Selaginella Kraussii (?) (Lycopo- 
diaceas). — A very young plant, only 
•4 of an inch in height, had sprung 
up in a pot in the hot-house. An 
extremely fine glass filament was 
fixed to the end of the frond-like stem, and the movement of 
the bead traced on a horizontal glass. It changed its course 
several times, as shown in Fig. 54, whilst observed during 13 
h. 15 m., and returned at night to a point not far distant from 
that whence it had started in the morning. There can be no 
doubt that this little plant circumnutated. 



Fig. 54. 

Selaginella Kraussii (?) : cir- 
cumuutation of young 
plant, kept in darkness, 
traced from 8.45 a.m. to 
10 P.M. Oct. 31st. 



CHAPTER II. 

General CoNsroERATioNS on the Movements and Growth 
OF Seedling Plants. 

Generality of the circumnutating movement — Eadicles, their circum- 
nutation of service — Manner in which they penetrate the ground 
— Manner in which hypocotyls and other organs break through 
the ground by being arched — Singular manner of germination in 
Megarrhiza, &c. — Abortion of cotyledons — Circumnutation of hypo- 
cotyls and epicotyls whilst still buried and arched — Their power 
of straightening themselves — Bursting of the seed-coats — Inherited 
effect of the arching process in hypogean hypocotyls — Circumnu- 
tation of hypocotyls and epicotyls when erect — Circumnutation of 
cotyledons — Pulvini or joints of cotyledons, duration of their activ- 
ity, rudimentary in Oxalis corniculata, their development — Sen- 
sitiveness of cotyledons to light and consequent disturbance of 
their periodic movements — Sensitiveness of cotyledons to contact. 

The circnmnutating movements of the several parts 
or organs of a considerable number of seedling plants 
have been described in the last chapter. A list is here 
appended of the Families, Cohorts, Sab-classes, &c., to 
which they belong, arranged and numbered according 
to the classification adopted by Hooker.* Any one who 
will consider this list will see that the young plants 
selected for observation, fairly represent the whole vege- 
table series excepting the lowest cryptogams, and the 
movements of some of the latter when mature will here- 
after be described. As all the seedlings which were 
observed, including Conifers, Cycads and Ferns, which 



* As given in tJic 'General System of Botany,' by Le Maout and 
Decaisne, 1873. 

69 



70 CIRCUMNUTATION OF SEEDLINGS. Chap. II. 

belong to the most ancient types amongst plants, were 
continually circumnutating, we may infer that this kind 
of movement is common to every seedling species. 

Sub-Kingdom I.— Phsenogamous Plants. 

Class I. — Dicotyledons. 

Sub-class I. — Angiosperms. 

Family. Cohort. 

14. Cruciferse. II. Parietales, 

26. Caryophyllege. IV. Caeyophyllales. 

36. Malvacese. VI. Malvales. 

41. Oxalidese. VII. Geraniales. 

49. Tropseolese. Ditto 

52. Aurantiacex. Ditto 

70. Hippocastanese. X. Sapindales. 

75. Leguminosx. XI. Eosales. 

106. Cucurhitacese. XII. Passiflorales. 

109. Cactese. XIV. Ficoidales. 

122. Compositse. XVII. Astrales. 

135. Primulacese. XX. Primulales. 

145. Asdepiadese. XXII. Gentianales. 

151. Convolvulacese. XXIII. Polemoniales. 

154. Borraginess. Ditto 

■ 156. Nolanese. Ditto 

157. Solanese. XXIV. Solanales. 

181. Chenopodiese. XXVII. Chenopodiales. 

202. Euphorbiacex. XXXII. Euphorbiales, 

211. Cupidiferse. XXXVI. Quernales. 

212. Corylacex. Ditto. 

Sub-class II. — Gymnosperms. 

223. Coniferx. 

224. Cycadex. 

Class II. — Monocotyledons. 

2. Cannacex. II. Amomales. 

34. Liliacex. XI. Liliales. 
41. Asparagex. Ditto 

55. Graminex. XV. Glumales. 

Sub-Kingdom II.— Cryptog-amio Plants. 
1. Filices. I. FilIcales. 

6. Lycopodiacex. Ditto 



Chap. II. ACTION OF THE RADICLE. 71 

Radicles. — In all the germinating seeds observed by 
us, the first change is the protrusion of the radicle, 
which immediately bends downwards and endeavours to 
penetrate the ground. In order to effect this, it is 
almost necessary that the seed should be pressed down so 
as to offer some resistance, unless indeed the soil is 
extremely loose ; for otherwise the seed is lifted up, 
instead of the radicle penetrating the surface. But 
seeds often get covered by earth thrown up by burrowing 
quadrupeds or scratching birds, by the castings of earth- 
worms, by heaps of excrement, the decaying branches of 
trees, &c., and will thus be pressed down ; and tliey must 
often fall into cracks when the ground is dry, or into 
holes. Even with seeds lying on the bare surface, the 
first developed root-hairs, by becoming attached to stones 
or other objects on the surface, are able to hold down 
the upper part of the radicle, whilst the tip penetrates 
the ground. Sachs has shown* how well and closely 
root-hairs adapt themselves by growth to the most irreg- 
ular particles in the soil, and become firmly attached to 
them. This attachment seems to be effected by the 
softening or liquefaction of the outer surface of the wall 
of the hair and its subsequent consolidation, as will be 
on some future occasion more fully described. This 
intimate union plays an important part, according to 
Sachs, in the absorption of water and of the inorganic 
matter dissolved in it. The mechanical aid afforded by 
the root-hairs in penetrating the ground is probably only 
a secondary service. 

The tip of the radicle, as soon as it protrudes from 
the seed-coats, begins to circumnutate, and the whole 
growing part continues to do so, probably for as long as 



* 'Physiologie Vegctalc,' 1868, pp. 199, 205. 
C 



Y2 ACTION OF THE RADICLE. Chap. II. 

growth continues. This movement of the radicle haa 
been described in Brassica, ^sculus, Phaseolus, Vicia, 
Cucnrbita, Quercus and Zea. The probability of its 
occurrence was inferred by Sachs,* from radicles placed 
yertically upwards being acted on by geotropism (which 
we likewise found to be the case), for if they had 
remained absolutely perpendicular, the attraction of 
gravity could not have caused them to bend to any one 
side. Circumnutation was observed in the above speci- 
fied cases, either by means of extremely fine filaments of 
glass affixed to the radicles in the manner previously 
described, or by their being allowed to grow downwards 
over inclined smoked glass-plates, on which they left 
their tracks. In the latter cases the serpentine course 
(see Figs. 19, 21, 27, 41) showed unequivocally that the 
apex had continually moved from side to side. This 
lateral movement was small in extent, being in the case 
of Phaseolus at most about 1 mm. from a medial line to 
both sides. But there was also movement in a vertical 
plane at right angles to the inclined glass-plates. This 
was shown by the tracks often being alternately a little 
broader and narrower, due to the radicles having alter- 
nately pressed with greater and less force on the plates. 
Occasionally little bridges of soot were left across the 
tracks, showing that the apex had at these spots been 
lifted up. This latter fact was especially apt to occur 
when the radicle instead of travelling straight down the 
glass made a semicircular bend ; but Fig. 52 shows that 



* ' Ueber das Wachsthum der it repeatedly. Dr. Frank had 

Wurzeln : Arbeiten des bot. In- previously remarked ( ' Beitriige 

stituts in Wurzburg,' Heft iii. zur Pflanzenphysiologie, 1868, p. 

1873, p. 460. This memoir, be- 81 ) on the fact of radicles placed 

sides its intrinsic and great in- vertically upwards being acted on 

terest, deserves to be studied as a by geotropism, and he explained 

model of careful investigation, and it by the supposition that their 

we shall have occasion to refer to growth was not equal on all sides. 



Chap. II. ACTION OF THE RADICLE. 73 

this may occur when the track is rectilinear. The apex 
by thus rising, was in one instance able to surmount a 
bristle cemented across an inclined glass-plate ; but slips 
of wood only ^^^ of an inch in thickness always caused 
the radicles to bend rectangularly to one side, so that the 
apex did not rise to this small height in opposition to 
geotropism. 

In those cases in which radicles with attached fila- 
ments were placed so as to stand up almost vertically, 
they curved downwards through the action of geotro- 
pism, circumnutating at the same time, and their courses 
were consequently zigzag. Sometimes, however, they 
made great circular sweeps, the lines being likewise 
zigzag. 

Radicles closely surrounded by earth, even when this 
is thoroughly soaked and softened, may perhaps be quite 
prevented from circumnutating. Yet we should remem- 
ber that the circumnutating sheath-like cotyledons of 
Phalaris, the hypocotyls of Solanum, and the epicotyls 
of Asparagus formed round themselves little circular 
cracks or furrows in a superficial layer of damp argilla- 
ceous sand. They were also able, as well as the hypo- 
cotyls of Brassica, to form straight furrows in damp 
sand, whilst circumnutating and bending towards a 
lateral light. In a future chapter it will be shown that 
the rocking or circumnutating movement of the flower- 
heads of Trifolium subterraneum aids them in burying 
themselves. It is therefore probable that the circumnu- 
tation of the tip of the radicle aids it slightly in pene- 
trating the ground ; and it may be observed in several of 
the previously given diagrams, that the movement is 
more strongly pronounced in radicles when they first 
protrude from the seed than at a rather later period; 
but whether this is an accidental or an adaptive coinci- 
dence we do not pretend to decide. Nevertheless, when 



74: ACTION OF THE RADICLE. Chap. II. 

young radicles of Phaseolus multifiorus were fixed verti- 
cally close over damp sand, in tlie expectation that as 
soon as they reached it they would form circular furrows, 
this did not occur, — a fact which may be accounted for, 
as we believe, by the furrow being filled up as soon as 
formed by the rapid increase of thickness in the apex of 
the radicle. Whether or not a radicle, when surrounded 
by softened earth, is aided in forming a passage for itself 
by circumnutating, this movement can hardly fail to be 
of high importance, by guiding the radicle along a line 
of least resistance, as will be seen in the next chapter 
when we treat of the sensibility of the tip to contact. 
If, however, a radicle in its downward growth breaks 
obliquely into any crevice, or a hole left by a decayed 
root, or one made by the larva of an insect, and more 
especially by worms, the circumnutating movement of 
the tip will materially aid it in following such open 
passage ; and we have observed that roots commonly run 
down the old burrows of worms.*^ 

When a radicle is placed in a horizontal or inclined 
position, the terminal growing part, as is well known, 
bends down towards the centre of the earth ; and Sachsf 
has shown that whilst thus bending, the growth of the 
lower surface is greatly retarded, whilst that of the upper 
surface continues at the normal rate, or may be even 
somewhat increased. He has further shown by attaching 
a thread, running over a pulley, to a horizontal radicle 
of large size, namely, that of the common bean, that it 
was able to pull up a weight of only one gramme, or 154 



* See, also, Professor Heiisen's means of the burrows made by- 
statements ( ' Zeitschrift fiir Wis- worms. 

sen. Zool.,' Band xxviii. page t ' Arbeiten des bot. Inst. Wiirz- 

354, 1877) to the same effect. He burg,' vol. i. 1873, p. 461. See 

goes so far as to believe that also p. 397 for the length of the 

roots are able to penetrate the growing part, and p. 451 on the 

ground to a great depth only by force of geotropism. 



Chap. 11. ACTION OP THE RADICLE. Y5 

grains. We may therefore conclude that geotropism 
does not give a radicle force sufficient to penetrate the 
ground, but merely tells it (if such an expression may 
be used) which course to pursue. Before we knew of 
Sachs' more precise observations we covered a flat surface 
of damp sand with the thinnest tin-foil which we could 
procure (-02 to -03 mm., or -00012 to -00079 of an inch 
in thickness), and placed a radicle close above, in such a 
position that it grew almost perpendicularly downwards. 
When the apex came into contact with the polished level 
surface it turned at right angles and glided over it 
without leaving any impression ; yet the tin-foil was so 
flexible, that a little stick of soft wood, pointed to the 
same degree as the end of the radicle and gently loaded 
with a weight of only a quarter of an ounce (120 grains) 
plainly indented the tin-foil. 

Eadicles are able to penetrate the ground by the 
force due to their longitudinal and transverse growth ; 
the seeds themselves being held down by the weight of 
the superincumbent soil. In the case of the bean the 
apex, protected by the root-cap, is sharp, and the growing 
part, from 8 to 10 mm. in length, is much more rigid, as 
Sachs has proved, than the part immediately above, 
which has ceased to increase in length. We endeavoured 
to ascertain the downward pressure of the growing part, 
by placing germinating beans between two small metal 
plates, the upper one of which was loaded with a known 
weight ; and the radicle was then allowed to grow into a 
narrow hole in wood, 2 or 3 tenths of an inch in depth, 
and closed at the bottom. The wood was so cut that the 
short space of radicle between the mouth of the hole and 
the bean could not bend laterally on three sides ; but it 
was impossible to protect the fourth side, close to the 
bean. Consequently, as long as the radicle continued 
to increase in length and remained straiglit, the weighted 



Y6 ACTION OF THE RADICLE. Chap. II. 

bean would be lifted up after the tip had reached the 
bottom of the shallow hole. Beans thus arranged, sur- 
rounded by damp sand, lifted up a quarter of a pound in 
24 h. after the tip of the radicle had entered the hole. 
With a greater weight the radicles themselves alwa3'S 
became bent on the one unguarded side ; but this prob- 
ably would not have occurred if they had been closely 
surrounded on all sides by compact 
_^^i__l^^-\ earth. There was, however, a possi- 
ble, but not probable, source of error 
in these trials, for it was not ascer- 

Outlinc of piece of , . , ■■ ,, j.i i .i ^ 

stick (reduced to tamed Avhether the beans themselves 




go on swelling for several days after 



one - half natural 
size) with a hole 

through which the they have germinated, and after hav- 

radicle of a bean , , j^ ^^ t • -i 

grew. Thickness of mg been treated m the manner m 

•08 Iniirat^'wd "^^^^^^ ^^^^ ^^^ ^^^^ ' namely, being 
end -16 ;' depth of first left for 24 h. in water, then al- 

hole *1 inch. , t , • i • i 

lowed to germinate m very damp air, 
afterwards placed over the hole and almost surrounded 
by damp sand in a closed box. 

We succeeded better in ascertaining the force exerted 
transversely by these radicles. Two were so placed as to 
penetrate small holes made in little sticks, one of which 
was cut into the shape here exactly copied (Fig. 55). 
The short end of the stick beyond the hole was purposely 
split, but not the opposite end. As the wood was highly 
elastic, the split or fissure closed immediately after being 
made. After six days the stick and bean were dug out 
of the damp sand, and the radicle was found to be much 
enlarged above and beneath the hole. The fissure, 
which was at first quite closed, was now open to a width 
of 4 mm. ; as soon as the radicle was extracted, it imme- 
diately closed to a width of 2 mm. The stick was then 
sns])ondod horizontally by a fine wire passing through 
tli(^ liolo lately filled by the radicle, and a little saucer 



Chap. II. 



ACTION OF THE RADICLE. 



T7 



was suspended beneath to receive the weights; and it 
required 8 lbs. 8 ozs. to open the fissure to the width of 
4 mm. — that is, the width before the root was extracted. 
But the part of the radicle (only -1 of an inch in length) 
which was embedded in the hole, probably exerted a 
greater transverse strain even than 8 lbs. 8 ozs., for it 
had split the solid wood for a length of rather more than a 
quarter of an inch (exactly -275 inch), and this fissure is 
shown in Fig. 55. A second stick was tried in the same 
manner with almost exactly the same result. 

We then followed a better 
plan. Holes were bored near 
the narrow end of two wooden 
clips or pincers (Fig. 56), kept 
closed by brass spiral springs. 
Two radicles in damp sand 
were allowed to grow through 
these holes. The pincers rest- 
ed on glass-plates to lessen the 
friction from the sand. The 
holes were a little larger (viz. 
•14 inch) and considerably 
deeper (viz. -6 inch) than in 
the trials Avith the sticks; so 
that a greater length of a 
rather thicker radicle exerted 
a transverse strain. After 13 
days they were taken up. The 
distance of two dots (see the 
figure) on the longer ends of 
the pincers was now carefully 
measured ; the radicles were 
then extracted from the holes, 
and the pincers of course closed. They were then sus- 
pended horizontally in the same manner as were the bits 




Wooden pincers, kept closed by 
a spiral brass spring, with a 
hole ("14 inch in diameter 
and "6 inch in depth) bored 
thronsh the narrow closed 
part, through which a radicle 
of a bean was allowed to 
grow. Temp. 50°-60° F. 



78 ACTION OF THE RADICLE. Ciiap. II. 

of sticks, and a weight of 1500 grains (or 3 lbs. 4 ozs.) 
was necessary with one of the pincers to open them to 
the same extent as had been effected by the transverse 
growth of the radicle. As soon as this radicle had 
slightly opened the pincers, it had grown into a flattened 
form and had escaped a little beyond the hole ; its diam- 
eter in one direction being 4-2 mm., and at right angles 
3-5 mm. If this escape and flattening could have been 
prevented the radicle would probably have exerted a 
greater strain than the 3 lbs. 4 ozs. With the other 
pincers the radicle escaped still further out of the hole ; 
and the weight required to open them to the same extent 
as had been effected by the radicle, was only 600 grams. 

With these facts before us, there seems little difficulty 
in understanding how a radicle penetrates the ground. 
The apex is pointed and is protected by the root-cap ; 
the terminal growing part is rigid, and increases in length 
with a force equal, as far as our observations can be 
trusted, to the pressure of at least a quarter of a pound, 
probably with a much greater force when prevented 
from bending to any side by the surrounding earth. 
Whilst thus increasing in length it increases in thickness, 
pushing away the damp earth on all sides, with a force 
of above 8 pounds in one case, of 3 pounds in another 
case. It was impossible to decide whether the actual 
apex exerts, relatively to its diameter, the same trans- 
verse strain as the parts a little higher up ; but there 
•seems no reason to doubt that this would be the case. 
^Mie growing part therefore does not act like a nail when 
hammered into a board, but more like a wedge of wood, 
which whilst slowly driven into a crevice continually ex- 
pands at tlie same time by the absorption of water ; and 
a wedge thus acting will split even a mass of rock. 

Manner in tvhich Hypocotyls, Epicofyls, dc, rise up 
and break' flirotigh the ground.— Xii^v the radicle has 



CiiAP. II. HYPOCOTYLS, EPICOTYLS, ETC. 



79 



Fig. 57. 



penetrated the ground and fixed the seed, the hypo- 
cotyls of all the dicotyledonous seedlings observed by 
us, which lift their cotyledons above the surface, break 
through the ground in the form of an arch. When the 
cotyledons are hypogean, that is, remain buried in the 
soil, the hypocotyl is hardly developed, and the epicotyl 
or plumule rises in like manner as an arch through the 
ground. In all, or at least in most of such cases, the 
downwardly bent apex remains for a time enclosed 
Avithin the seed-coats. With Corylus avellcna the cotyle- 
dons are hypogean, and the epicotyl is arched ; but in 
the particular case described in the last chapter its apex 
had been injured, and it grew later- 
ally through the soil like a root ; and 
in consequence of this it had emitted 
two secondary shoots, which likewise 
broke through the ground as arches. 

Cyclamen does not produce any 
distinct stem, and only a single cotyle- 
don appears at first ; * its petiole 
breaks through the ground as an arch 
(Fig. 57). Abronia also has only a 
single fully developed cotyledon, but 
in this case it is the hypocotyl which 
first emerges and is arched. Abronia 
umiellata^ however, presents this pe- 
culiarity, that the enfolded blade of 
the one developed cotyledon (with the 
enclosed endosperm) whilst still beneath the surface has 
its apex upturned and parallel to the descending leg of 
the arched hypocotyl ; but it is dragged out of the 




Cyclamen Persicum : 
seedling, figure en- 
larged : c, blade 
of cotyledon, not 
yet expanded, with 
arched petiole be- 
ginning to straight- 
en itself; h, hypo- 
cotyl developed in- 
to a corra ; r, sec- 
ondary radicles. 



* This is the conclusion arrived ered by other botanists as the 
at by Dr. H. Gressner ( ' Bot. first true leaf is really the second 
Zoitung,' 1874, p. 837), who main- cotyledon, which is greatly de- 
tains that what has been consid- layed in its development. 



80 



HYPOCOTYLS, EPICOTYLS, ETC. Chap. II. 



ground by the continued growth of the hypocotyl, with 
the apex pointing downward. With Cycas pectiiiata the 
cotyledons are hypogean, and a true leaf first breaks 
through the ground with its petiole forming an arch. 

In the genus Acanthus the cotyledons are likewise 
hypogean. In A. mollis, a single leaf first breaks 
through the ground with its petiole arched, and with the 
opposite leaf much less developed, short, straight, of a 

yellowish colour, and with 
Fig. 58. ^ ^^^ petiole at first not half 

as thick as that of the 
other. The undeveloped 
leaf is protected by stand- 
ing beneath its arched fel- 
low ; and it is an instruc- 
tive fact that it is not 
arched, as it has not to 
force for itself a passage 
through the ground. In 
the accompanying sketch 
(Fig. 58) the petiole of the 
first leaf has already par- 
tially straightened itself, 
and the blade is beginning 
to unfold. The small sec- 
ond leaf ultimately grows 
to an equal size with the 
first, but this process is effected at very different rates in 
different individuals : in one instance the second leaf did 
not appear fully above the ground until six weeks after 
tlie first leaf. As the leaves in the whole family of the 
AcanthaceiT3 stand either opposite one another or in 
whorls, and as these are of equal size, the great inequal- 
ity between the first two leaves is a singular fact. We 
can see how this inequality of development and the 




Acanthus mollis : seedling, with the 
liypogcan cotyledon on the near 
side removed and the radicles cut 
off: a, blade of first leaf begin- 
ning to expand, with petiole still 
partially arched ; 6, second and 
opposite leaf, as yet very imper- 
fectly developed ; c, hypogean 
cotyledon on the opposite side. 



Chap. II. BREAKING THROUGH THE GROUND. 81 

arching of the petiole could have been gradually ac- 
quired, if they were beneficial to the seedlings by favour- 
ing their emergence ; for with A. candelabrum, spinosuSy 
and latifolius there was great variability in the inequal- 
ity between the two first leaves and in the arching of 
their petioles. In one seedling of A. candelabrum the 
first leaf was arched and nine times as long as the sec- 
ond, which latter consisted of a mere little, yellowish- 
white, straight, hairy style. In other seedlings the dif- 
ference in length between the two leaves was as 3 to 2, or 
as 4 to 3, or as only -76 to "62 inch. In these latter cases 
the first and taller leaf was not properly arched. Lastly, 
in another seedlinsr there was not the least difference in 
size between the two first leaves, and both of them had 
their petioles straight ; their laminae were enfolded and 
pressed against each other, forming a lance or wedge, 
by which means they had broken through the ground. 
Therefore in different individuals of this same species 
of Acanthus the first pair of leaves breaks through the 
ground by two widely different methods ; and if either 
had proved decidedly advantageous or disadvantageous, 
one of them no doubt would soon have prevailed. 

Asa Gray has described* the peculiar manner of 
germination of three widely different plants, in which 
the hypocotyl is hardly at all developed. These were 
therefore observed by us in relation to our present sub- 
ject. 

Delphinium nudicaule.~The elongated petioles of 
the two cotyledons are confluent (as are sometimes their 
blades at the base), and they break through the ground 
as an arch. They thus resemble in a most deceptive 
manner a hypocotyl. At first they are solid, but after a 
time become tubular; and the basal part beneath the 



'Botanical Text-Book,' 1879, p. 22. 



82 IIYPOCOTYLS, EPICOTYLS, ETC. Chap. II. 

ground is enlarged into a hollow chamber, within which 
the young leaves are developed without any prominent 
plumule. Externally root-hairs are formed on the con- 
fluent petioles, either a little above, or on a level" with, 
the plumule. The first leaf at an early period of its 
growth and whilst within the chamber is quite straight, 
but the petiole soon becomes arched ; and the swelling 
of this part (and probably of the blade) splits open one 
side of the chamber, and the leaf then emerges. The 
slit was found in one case to be 3-2 mm. in length, and 
it is seated on the line of confluence of the two petioles. 
The leaf when it first escapes from the chamber is buried 
beneath the ground, and now an upper part of the peti- 
ole near the blade becomes arched in the usual manner. 
The second leaf comes out of the slit either straight or 
somewhat arched, but afterwards the upper part of the 
petiole, — certainly in some, and we believe in all cases, — 
arches itself whilst forcing a passage through the soil. 

Megarrhiza Californica. — The cotyledons of this 
Oourd never free themselves from the seed-coats and 
are hypogean. Their petioles are completely confluent, 
forming a tube which terminates downv/ards in a little 
solid point, consisting of a minute radicle and hypocotyl, 
with the likewise minute plumule enclosed within the 
base of the tube. This structure was well exhibited in 
an abnormal specimen, in which one of the two cotyle- 
dons failed to produce a petiole, whilst the other pro- 
duced one consisting of an open semicylinder ending 
in a sharp point, formed of the parts just described. As 
soon as the confluent petioles protrude from the seed they 
bend down, as they are strongly geotropic, and penetrate 
the ground. The seed itself retains its original position, 
either on the surface or buried at some depth, as the 
case may b(^ If, however, the point of the confluent 
pctiok's meets with some obstacle in the soil, as appears 



Chap. II. BREAKING THROUGH THE GROUND. 83 

to have occurred with the seedlings described and figured 
by Asa Gray,* the cotyledons are lifted up above the 
ground. The petioles are clothed with root-hairs like 
those on a true radicle, and they likewise resemble 
radicles in becoming brown when immersed in a solution 
of permanganate of potassium. Our seeds were sub- 
jected to a high temperature, and in the course of three 
or four days the petioles penetrated the soil perpendicu- 
larly to a depth of from 2 to 2^ inches ; and not until 
then did the true radicle begin to grow. In one speci- 
men which was closely observed, the petioles in 7 days 
after their first protrusion attained a length of 2^ inches, 
and the radicle by this time had also become well 
developed. The plumule, still enclosed within the tube, 
was now -3 inch in length, and was quite straight ; but 
from having increased in thickness it had just begun to 
split open the lower part of the petioles on one side, 
along the line of their confluence. By the following 
morning the upper part of the plumule had arched itself 
into a right angle, and the convex side or elbow had thus 
been forced out through the slit. Here then the arching 
of the plumule plays the same part as in the case of the 
petioles of the Delphinium. As the plumule continued 
to grow, the tip became more arched, and in the course 
of six days it emerged through the 2| inches of superin- 
cumbent soil, still retaining its arched form. After 
reaching the surface it straightened itself in the usual 
manner. In the accompanying figure (Fig. 58, A) we 
have a sketch of the seedling in this advanced state of 
development; the surface of the ground being repre- 
sented by the line G- . . . G. 

The germination of the seeds in their native Califor- 
nian home proceeds in a rather d liferent manner, as we 



*^ ' American Journal of Science,' vol. xiv. 1877, p. 21. 



84 



HYPOCOTYLS, EPICOTYLS, ETC. Chap. II. 



58, A. 



infer from an interesting letter from Mr. Eattan, sent to 
us by Prof. Asa Gray. The petioles protrude from the 
seeds soon after the autumnal rains, and penetrate the 
ground, generally in a vertical direc- 
tion, to a depth of from 4 to even 
6 inches. They were found in this 
state by Mr. Rattan during the 
Christmas vacation, with the plu- 
mules still enclosed within the tubes; 
and he remarks that if the plumules 
had been at once developed and had 
reached the surface (as occurred 
with our seeds which were exposed 
to a high temperature), they would 
surely have been killed by the frost. 
As it is they lie dormant at some 
depth beneath the surface, and are 
thus protected from the cold ; and 
the root-hairs on the petioles would 
supply them with sufficient moist- 
ure. We shall hereafter see that 
many seedlings are protected from 
frost, but by a widely different pro- 
cess, namely, by being drawn be- 
neath the surface by the contraction 
of their radicles. We may, how- 
ever, believe that the extraordinary 
manner of germination of Megar- 
rhiza has another and secondary ad- 
vantage. The radicle begins in a few weeks to enlarge 
into a little tuber, which then abounds with starch and 
is only slightly bitter. It would therefore be very liable 
to be devoured by animals, were it not protected by being 
buried wliilst young and tender, at a depth of some inches 
beneath the surface. Ultimately it grows to a huge size. 




Meoarrhisa Californica : 
sketch of seedling, 
copied from Asa Gray, 
reduced to one -half 
scale : c, cotyledons 
within seed-c6ats ; p, 
the two confluent 
petioles ; h and r, hy- 
pocotyl and radicle ; 

pi, i)hnnulc; G (?, 

surface of soil. 



Chap. II. BREAKING THROUGH THE GROUND. 85 

Ipomoea leptophylla. — In most of the species of this 
genus the hypocotyl is well developed, and breaks through 
the ground as an arch. But the seeds of the present 
species in germinating behave like those of Megarrhiza, 
excepting that the elongated petioles of the cotyledons 
are not confluent. After they have protruded from the 
seed, they are united at their lower ends with the unde- 
veloped hypocotyl and undeveloped radicle, which to- 
gether form a point only about -1 inch in length. They 
are at first highly geotropic, and penetrate the ground to 
a depth of rather above half an inch. The radicle then 
begins to grow. On four occasions after the petioles 
had grown for a short distance vertically downwards, 
they were placed in a horizontal position in damp air in 
the dark, and in the course of 4 hours they again became 
curved vertically downwards, having passed through 90° 
in this time. But their sensitiveness. to geotropism lasts 
for only 2 or 3 days; and the terminal part alone, for 
a length of between '2 and '4 inch, is thus sensitive. 
Although the petioles of our specimens did not penetrate 
the ground to a greater depth than about -| inch, yet 
they continued for some time to grow rapidly, and finally 
attained the great length of about 3 inches. The upper 
■part is apogeotropic, and therefore grows vertically 
upwards, excepting a short portion close to the blades, 
which at an early period bends downwards and becomes 
arched, and thus breaks through the ground. After- 
wards this portion straightens itself, and the cotyledons 
then free themselves from the seed-coats. Thus we here 
have in different parts of the same organ widely different 
kinds of movement and of sensitiveness ; for the basal 
part is geotropic, the upper part apogeotropic, and a 
portion near the blades temporarily and spontaneously 
arches itself. The plumule is not developed for some 
little time ; and as it rises between the bases of the par- 



86 IIYPOCOTYLS, EPICOTYLS, ETC. Chap. II. 

allel and closely approximate petioles of the cotyledons, 
which in breaking through the ground have formed an 
almost open passage, it does not require to be arched 
and is consequently always straight. Whether the plu- 
mule remains buried and dormant for a time in its native 
country, and is thus protected from the cold of winter, 
we do not know. The radicle, like that of the Megar- 
rhiza, grows into a tuber-like mass, which ultimately 
attains a great size. So it is with Ipomma pmidurata^ 
the germination of which, as Asa Gray informs us, 
resembles that of /. leptojyhylla. 

The following case is interesting in connection with 
the root-like nature of the petioles. The radicle of a 
seedling was cut off, as it was completely decayed, and 
the two now separated cotyledons were planted. They 
emitted roots from their bases, and continued green and 
healthy for two months. The blades of both then 
withered, and on removing the earth the bases of the 
petioles (instead of the radicle) were found enlarged into 
little tubers. Whether these would have had the power 
of producing two independent plants in the following 
summer, we do not know. 

In QuerciLS virens, according to Dr. Engelmann,* 
both the cotyledons and their petioles are confluent. 
The latter grow to a length " of an inch or even more ; " 
and, if we understand rightly, penetrate the ground, so 
that they must be geotropic. The nutriment within the 
cotyledons is then quickly transferred to the hypocotyl 
or radicle, which thus becomes developed into a fusiform 
tuber. Tlie fact of tubers being formed by the forego- 
ing three widely distinct plants, makes us believe that 
tlicir ])rotection from animals at an early age and whilst 
tender, is one at least of the advantages gained by the 



Tninsuct. St. Louis Acad. Science,' vol. iv. p. 190. 



Chap. II. BREAKING THROUGH THE GROUND. 



87 



remarkable elongation of the petioles of the cotyledons, 
together with their power of penetrating the ground like 
roots under the guidance of geotropism. 



The following cases may be here given, as they bear 
on our present subject, though not relating to seedlings. 
The flower-stem of the parasitic Lathrcea squamaria^ 
which is destitute of true leaves, breaks through the 
ground as an arch;* so does the flower-stem of the 
parasitic and leafless Motwtropa hypopitys. With Helle- 
dorus niger^ the flower-stems, which rise up independ- 
ently of the leaves, likewise break through the ground 
as arches. This is also the case with the greatly elongated 
flower-stems, as well as with the petioles of Epimedium 
pinnatum. So it is with the petioles of Ranunculus 



* The passage of the flower-stein 
of the Lathrsea through the ground 
cannot fail to be greatly facilitated 
by the extraordinary quantity of 
water secreted at this period of the 
year by the subterranean scale-like 
leaves ; not that there is any reason 
to suppose that the secretion is a 
special adaptation for this purpose : 
it probably follows from the great 
quantity of sap absorbed in the 
early spring by the parasitic roots. 
After a long period without any 
rain, the earth had become light- 
coloured and very di'y, but it was 
dark-coloured and damp, even in 
parts quite wet, for a distance of 
at least six inches all round each 
floAver-stem. The water is se- 
creted by glands (described by 
Colin, ' Bericht. Bot. Sect, der 
Schlesischen Gesell.,' 1876, p. 113) 
which line the longitudinal chan- 
nels running through each scale- 
like leaf A large plant was dug 
up, washed so as to remove the 
earth, left for some time to drain, 
and then placed in the evening on 
a dry glass-plate, covered with a 
bell-glass, and by next morning it 
had secreted a large pool of water. 
7 



The plate was wiped dry, and in 
the course of the succeeding 7 or 
8 hours another little pool was se- 
creted, and after 16 additional hours 
several large drops. A smaller 
plant was washed and placed in a 
large jar, which was left inclined 
for an hour, by which time no 
more water drained off. The jar 
was then placed upright and 
closed : after 23 hours two drachms 
of water were collected from the 
bottom, and a little more after 25 
additional hours. The flower- 
stems were now cut off, for they do 
not secrete, and the subterranean 
part of the plant Avas found to 
weigh 106 "8 grams (1611 grains), 
and the water secreted during 
the 48 hours weighed 11'9 grams 
(183 grains), — that is, one-ninth 
of the whole weight of the plant, 
excluding the flower-stems. We 
should remember that plants in 
a state of nature would probably 
secrete in 48 hours much more 
than the above large amount, 
for their roots would continue all 
the time absorbing sap from the 
plant on which they were para- 
sitic. 



88 HYPOCOTYLS, EPICOTYLS, ETC. Chap. II. 

ficaria, when they have to break through the ground, 
but when they arise from the summit of the bulb above 
ground, they are from the first quite straight; and 
this is a fact which deserves notice. The rachis of 
the bracken fern [Pteris aquilma), and of some, prob- 
ably many, other ferns, likewise rises above ground 
under the form of an arch. 'No doubt other analogous 
instances could be found by careful search. In all 
ordinary cases of bulbs, rhizomes, root-stocks, &c., buried 
beneath the ground, the surface is broken by a cone 
formed by the young imbricated leaves, the combined 
growth of which gives them force sufficient for the 
purpose. 

With germinating monocotyledonous seeds, of which, 
however, we did not observe a large number, the plumules, 
for instance, those of Asparagus and Canna, are straight 
whilst breaking through the ground. With the Grami- 
neae, the sheath-like cotyledons are likewise straight; 
they, however, terminate in a sharp crest, which is white 
and somewhat indurated; and this structure obviously 
facilitates their emergence from the soil : the first true 
leaves escape from the sheath through a slit beneath the 
chisel-like apex and at right angles to it. In the case of 
the onion {Allium cepa) we again meet with an arch ; the 
leaf-like cotyledon being abruptly bowed, when it breaks 
through the ground, with the apex still enclosed within 
the seed-coats. The crown of the arch, as previously 
described, is developed into a white conical protuberance, 
which we may safely believe to be a special adaptation 
for this office. 

The fact of so many organs of different kinds— hypo- 
cotyls and epicotyls, the petioles of some cotyledons and 
of some first leaves, the cotyledons of the onion, the 
rachis of some ferns, and some flower-stems — being all 
arched whilst thoy break through the ground, shows how 



Chap. ir. BREAKING THROUGH THE GROUND. 89 

just are Dr. Haberlandt's * remarks on the importance 
of the arch to seedling plants. He attributes its chief 
importance to the upper, young, and more tender parts 
of the hypocotyl or epicotyl, being thus saved from ab- 
rasion and pressure whilst breaking through the ground. 
But we think that some importance may be attributed 
to the increased force gained by the hypocotyl, epicotyl, 
or other organ by being at first arched ; for both legs of 
the arch increase in length, and both have points of re- 
sistance as long as the tip remains enclosed within the 
seed-coats ; and thus the crown of the arch is pushed up 
through the earth with twice as much force as that which 
a straight hypocotyl, &c., could exert. As soon, how- 
ever, as the upper end has freed itself, all the work has 
to be done by the basal leg. In the case of the epicotyl 
of the common bean, ^he basal leg (the apex having 
freed itself from the seed-coats) grew upwards with a 
force sufficient to lift a thin plate of zinc, loaded with 12 
ounces. Two more ounces were added, and the 14 
ounces were lifted up to a very little height, and then 
the epicotyl yielded and bent to one side. 

With respect to the primary cause of the arching 
process, we long thought in the case of many seedlings 
that this might be attributed to the manner in which the 
hypocotyl or epicotyl was packed and curved within the 
seed-coats; and that the arched shape thus acquired was 
merely retained until the parts in question reached the 
surface of the ground. But it is doubtful whether this 
is the whole of the truth in any case. For instance, with 
the common bean, the epicotyl or plumule is bowed into 
an arch whilst breaking through the seed-coats, as shown 



* ' Die Schutzcinrichtunn;en in though our ohscrvations lead us 

der Entwickelung der Keim- to differ on some points from the 

pflanze,' 1877. We have learned author, 
much from this interesting essay, 



90 HYPOCOTYLS, EPICOTYLS, ETC. Chap. II. 

in Fig. 59 (p. 93). The plumule first protrudes as a 
solid knob {e in A), which after twenty -four hours' 
growth is seen (e in B) to be the crown of an arch. 
Nevertheless, with several beans which germinated in 
damp air, and had otherwise been treated in an unnatu- 
ral manner, little plumules were developed in the axils 
of the petioles of both cotyledons, and these were as per- 
fectly arched as the normal plumule ; yet they had not 
been subjected to any confinement or pressure, for the 
seed-coats were completely ruptured, and they grew in 
the open air. This proves that the plumule has an in- 
nate or spontaneous tendency to arch itself. 

In some other cases the hypocotyl or epicotyl pro- 
trudes from the seed at first only slightly bowed ; but 
the bowing afterwards increases independently of any 
constraint. The arch is thus made narrow, with the 
two legs, which are sometimes much elongated, parallel 
and close together, and thus it becomes well fitted for 
breaking through the ground. 

With many kinds of plants, the radicle, whilst still 
enclosed within the seed and likewise after its first pro- 
trusion, lies in a straight line with the future hypocotyl 
and with the longitudinal axis of the cotyledons. This 
is the case with Cucurbita ovifera ; nevertheless, in 
whatever position the seeds were buried, the hypocotyl 
always came up arched in one particular direction. 
Seeds were planted in friable peat at a depth of about 
an inch in a vertical position, with the end from which 
the radicle protrudes downwards. Therefore all the 
parts occupied the same relative positions which they 
would ultimately hold after the seedlings had risen clear 
above the surface. Notwithstanding this fact, the 
hypocotyl arched itself; and as the arch grew upwards 
through the peat, tlie buried seeds were Uirned either 
uj)side down, or were laid horizontally, being afterwards 



Chap. II. BREAKING THROUGH THE GROUND. 91 

dragged above the ground. Ultimately the hypocotyl 
straightened itself in the usual manner ; and now after 
all these movements the several parts occupied the same 
position relatively to one another and to the centre of the 
earth, which they had done when the seeds were first 
buried. But it may be argued in this and other such 
cases that, as the hypocotyl grows up through the soil, 
the seed will almost certainly be tilted to one side ; and 
then from the resistance which it must offer during its 
further elevation, the upper part of the hypocotyl will 
be doubled down and thus become arched. This view 
seems the more probable, because with Ranunculus 
ficaria only the petioles of the leaves which forced a 
passage through the earth were arched ; and not those 
which arose from the summits of the bulbs above the 
ground. Nevertheless, this explanation does not apply 
to the Cucurbita, for when germinating seeds were sus- 
pended in damp air in various positions by pins passing 
through the cotyledons, fixed to the inside of the lids of 
jars, in which case the hypocotyls were not subjected to 
any friction or constraint, yet the upper part became 
spontaneously arched. This fact, moreover, proves that 
it is not the weight of the cotyledons which causes the 
arching. Seeds of Helianthus annuus and of two 
species of Ipomoea (those of /. hona nox being for the 
genus large and heavy) were pinned in the same manner, 
and the hypocotyls became spontaneously arched ; the 
radicles, which had been vertically dependent, assumed 
in consequence a horizontal position. In the case of 
Ipomoea le]jtop]iylla it is the petioles of the cotyledons 
which become arched whilst rising through the ground ; 
and this occurred spontaneously when the seeds were 
fixed to the lids of jars. 

It may, however, be suggested with some degree of 
probability that the arching was aboriginally caused by 



92 HYPOCOTYLS, EPICOTYLS, ETC. Chap. II. 

mechanical compulsion, owing to the confinement of the 
parts in question within the seed-coats, or to friction 
whilst they were being dragged upwards. But if this is 
so, we must admit from the cases just given, that a ten- 
dency in the upper part of the several specified organs to 
bend downwards and thus to become arched, has now 
become with many plants firmly inherited. The arching, 
to whatever cause it may be due, is the result of modified 
circumnutation, through increased growth along the 
convex side of the part ; such growth being only tem- 
porary, for the part always straightens itself subsequently 
by increased growth along the concave side, as will here- 
after be described. 

It is a curious fact that the hypocotyls of some plants, 
which are but little developed and which never raise 
their cotyledons above the ground, nevertheless inherit a 
slight tendency to arch themselves, although this move- 
ment is not of the least use to them. We refer to a 
movement observed by Sachs in the hypocotyls of the 
bean and some other Legumiuosie, and which is shown 
in the accompanying figure (Fig. 59), copied from his 
Essay.* The hypocotyl and radicle at first grow perpen- 
dicularly downwards, as at A, and then bend, often in 
the course of 24 hours, into the position shown at B. As 
we shall hereafter often have to recur to this movement, 
we will, for brevity sake, call it " Sachs' curvature." At 
first sisfht it mio-ht be thouo-ht that the altered position 
of the radicle in B was wholly due to the outgrowth of 
the epicotyl (e), the petiole (;;) serving as a hinge ; and 
it is probable that tliis is partly the cause ; but the hypo- 
cotyl and upper part of the radicle themselves become 
sliglitly curved. 

The above movement in the bean was repeatedly seen 



Arbciton dos hot. Instit. Wiirzburg,' vol. i 1873, p. 403. 



Chap. II. BREAKINQ THROUGH THE GROUND. 



93 



by us ; but our observations were made chiefly on Fha- 
seolus muUifiorus, the cotyledons of which are likewise 
hypogean. Some seedlings with well-developed radicles 
were first immersed in a solution of permanganate of 
potassium; and, judging from the changes of colour 
A B 

Fig. 59. 




Vicia faba : germinating seeds, suspended in damp air : A, with radicle 
growing perpendicularly downwards ; B, the same bean after 24 
hours and after the radicle has curved itself ; r, radicle ; h, short 
hypocotyl ; e, epicotyl appearing as a knob in A and as an arch in 
B ; p, petiole of the cotyledon, the latter enclosed within the seed- 
coats. 

(though these were not very clearly defined), the hypo- 
cotyl is about -3 inch in length. Straight, thin, black 
lines of this length were now drawn from the bases of 
the short petioles along the hypocotyls of 23 germinating 
seeds, which were pinned to the lids of jars, generally 
with the hilum downwards, and* with their radicles 
pointing to the centre of the earth. After an interval of 
from 24 to 48 hours the black lines on the hypocotyls of 
16 out of the 23 seedlings became distinctly curved, but 



94 HYPOCOTYLS, EPICOTYLS, ETC. Chap. II. 

in very various degrees (namely, with radii between 20 
and 80 mm. on Sachs' cyclometer) in the same relative 
direction as shown at B in Fig. 59. As geotropism will 
obviously tend to check this curvature, seven seeds were 
allowed to germinate with proper precautions for their 
growth in a klinostat,* by which means geotropism was 
eliminated. The position of the hypocotyls was observed 
during four successive days, and they continued to bend 
towards the hilum and lower surface of the seed. On 
the fourth day they were deflected by an average angle 
of 63° from a line perpendicular to the lower surface, 
and were therefore considerably more curved than the 
hypocotyl and radicle in the bean at B (Fig. 59), though 
in the same relative direction. 

It v/ill, we presume, be admitted that all leguminous 
plants with hypogean cotyledons are descended from 
forms which once raised their cotyledons above the 
ground in the ordinary manner ; and in doing so, it is 
certain that their hypocotyls would have been abruptly 
arched, as in the case of every other dicotyledonous 
plant. This is especially clear in the case of Phaseolus, 
for out of five species, the seedlings of which we observed, 
namely, P. mtiUiflorus, caracaUa, vulgaris, Hernandesii 
and Roxbnrgliii (inhabitants of the old and New Worlds), 
the three last-named species have well-developed hypo- 
cotyls which break through the ground as arches. Now, 
if we imagine a seedling of the common bean or of P. 
multiflorus, to behave as its progenitors once did, the 
hypocotyl (7i, Fig. 59), in whatever position the seed 
may have been buried, would become so much arched 
that the upper part would be doubled down parallel to 
the lower part ; and this is exactly the kind of curvature 



*An iiistrumont dovisod hy on which the plant under observa- 
Siiclis, consistinf,' cssontiiilly ol" a tioii is supported : see ' Wiirzburg 
Rlowly revolving' horizontal axis, Arbciten,' 1879, p. 209. 



Chap. II. 



RUDIMENTARY COTYLEDONS. 



95 



which actually occurs in these two plants, though to a 
much less degree. Therefore we can hardly doubt that 
their short hypocotyls have retained by inheritance a 
tendency to curve themselves in the same manner as 
they did at a former period, when this movement was 
highly important to them for breaking through the 
ground, though now rendered useless by the cotyledons 
being hypogean. Eudimentary structures are in most 
cases highly variable, and we might expect that rudi- 
mentary or obsolete actions would* be equally so; and 
Sachs' curvature varies extremely in amount, and some- 
times altogether fails. This is the sole instance known 
to us of the inheritance, though in a feeble degree, of 
movements which have become superfluous from changes 
which the species has undergone. 



Rudimentary Cotyledons. — A few remarks on this 
subject may be here interpolated. It is well known that 
some dicotyledonous ^ 
plants produce only a ^^ /Ac ^^' 
single cotyledon ; for 
instance, certain spe- 
cies of Eanunculus, 
Corydalis, Chserophyl- 
lum ; and we will here 
endeavour to show that 
the loss of one or both 
cotyledon is apparently 
due to a store of nutri- 
ment being laid up in 
some other part, as in 
the hypocotyl or one of 
the two cotyledons, or 
one of the secondary radicles. With the orange {Citrus 
aurantium) the cotyledons are hypogean, and one is 





Citrus auranthm. : two young seedlings : 
c, larger cotyledon; c', smaller coty- 
ledon ; h, thickened hypocotyl ; r, 
radicle. In A the epicotyl is still 
arched, in B it has become erect. 



96 



RUDIMENTARY COTYLEDONS. 



Chap. II. 



Fie. 61. 



larger than the other, as may be seen in A (Fig. 60). 
In B the inequality is rather greater, and the stem has 
grown between the points of insertion of the two petioles, 
so that they do not stand opposite to one another; in 
another case the separation amounted to one-fifth of an 
inch. The smaller cotyledon of one seedling was ex- 
tremely thin, and not half the length of the larger one, 
so that it was clearly becoming ru- 
dimentary.* In all these seedlings 
the hypocotyl was enlarged or 
swollen. 

With Ahronia umhellata one of 
the cotyledons is quite rudimen- 
tary, as may be seen {c') in Fig. 61. 
In this specimen it consisted of 
a little green flap, -g^th inch in 
length, destitute of a petiole and 
covered with glands like those on 
the fully developed cotyledon (c). 
At first it stood opposite to the 
larger cotyledon ; but as the petiole 
of the latter increased in length 
and gre\^*in the same line with the 
hypocotyl {h), the rudiment ap- 
peared in older seedlings as if 
seated some way down the hypo- 
cotyl. With Ahro^iia arenaria 
there is a similar rudiment, which in one specimen was 
oi^ly lio th and in another -giyth inch in length ; it nlti- 




Abronia nmbellata : seed- 
ling twice natural size : 
c, cotyledon ; c', rudi- 
mentary cotyledon ; h, 
enlarged hypocotyl, 
with a heel or projec- 
tion {h') at the lower 
end ; r, radicle. 



* In PacJiirn. nqiKitica, as de- 
scribed l)y Mr. R. I, Lyncl\ (' Jour- 
nal lAuu. Soc. Hot' vol. xvii. 1878, 
]). 1 17), one of th(> hypogetm coty- 
irdons is of inin»ens(> size ; the 
other is small and soon falls otl'; 
the pair do not always stand oj)- 
posito. In another and very dif- 



ferent water-plant, Trapa natans, 
one of the cotyledons, filled Avith 
farinaceous matter, is much larger 
than the other, which is scai-cely 
visible, as is stated by Aug. de 
CandoUe, ' Physiologie Veg.' torn, 
ii. p. 834, 1833. 



Chap. II. RUDIMENTARY COTYLEDONS. 97 

mately appeared as if seated halfway down the hypo- 
cotyl. In both these species the hypocotyl is so much 
enlarged, especially at a very early age, that it might 
almost be called a corm. The lower end forms a heel or 
projection, the use of which will hereafter be described. 

In Cyclamen Persicum the hypocotyl, even whilst 
still within the seed, is enlarged into a regular corm,* 
and only a single cotyledon is at first developed (see 
former Fig. 57). With Ranunculus ficaria two coty- 
ledons are never produced, and here one of the secondary 
radicles is developed at an early age into a so-called bulb.f 
Again, certain species of Ch^rophyllum and Corydalis 
produce only a single cotyledon \X in the former the 
hypocotyl, and in the latter the radicle is enlarged, 
according to Irmisch, into a bulb. 

In the several foregoing cases one of the cotyledons 
is delayed in its development, or reduced in size, or 
rendered rudimentary, or quite aborted ; but in other 
cases both cotyledons are represented by mere rudiments. 
With Opuntia hasilaris this is not the case, for both 
cotyledons are thick and large, and the hypocotyl shows 
at first no signs of enlargement; but afterwards, when 
the cotyledons have withered and disarticulated them- 
selves, it becomes thickened, and from its tapering form, 
together with its smooth, tough, brown skin, appears, 
when ultimately drawn down to some depth into the soil, 
like a root. On the other hand, with several Cacteae, the 
hypocotyl is from the first much enlarged, and both 
cotyledons are almost or quite rudimentary. Thus with 



*" Dr. H. Grcssncr, 'Bot. Zei- Vaucher's account ('Hist. Phvs. 

tung,' 1874, p. 824. des Plantes d'Europe,' torn. i. 1841, 

t Irmisch, ' Beitriigc zur Mor- p. 149) of the germination of the 

phologie der Pflaazen,' 1854, pp. seeds of several species of Cory- 

11, 12; 'Bot. Zeitm-ig,'1874, p. 805. dalis, that the bulb or tubercle 

t Delpino, ' Rivista Botanica,' begins to be formed at au ex- 

1877, p. 21. It is evident from tremely early age. 



98 RUDIMENTARY COTYLEDONS. Chap. II. 

Cereus LandhecTcii two little triangular projections, 
representing the cotyledons, are narrower than the hypo- 
cotyl, which is pear-shaped, with the point downwards. 
In RMpsalis cassytlia the cotyledons are represented by 
mere points on the enlarged hypocotyl. In Ecliinocactus 
viridesce7is the hypocotyl is globular, with two little 
promiuences on its summit. In Pilocereus Houlletii 
the hypocotyl, much swollen in the upper part, is merely 
notched on the summit ; and each side of the notch evi- 
dently represents a cotyledon. Stapelia sarpedon^ a 
member of the very distinct family of the Asclepiade^, 
is fleshy like a cactus ; and here again the upper part of 
the flattened hypocotyl is much thickened and bears tv70 
minute cotyledons, which, measured internally, were 
only -15 inch in length, and in breadth not equal to one- 
fourth of the diameter of the hypocotyl in its narrow 
axis ; yet these minute cotyledons are probably not quite 
useless, for when the hypocotyl breaks through the 
ground in the form of an arch, they are closed or pressed 
against one another, and thus protect the plumule. 
They afterwards open. 

From the several cases now given, which refer to 
widely distinct plants, we may infer that there is some 
close connection between the reduced size of one or both 
cotyledons and the formation, by the enlargement of the 
hypocotyl or of the radicle, of a so-called bulb. But it 
may be asked, did the cotyledons first tend to abort, or 
did a bulb first begin to be formed ? As all dicotyledons 
naturally produce two w^ell developed cotyledons, whilst 
the thickness of the hypocotyl and of the radicle differs 
much in different plants, it seems probable that these 
hitter organs first became from some cause thickened — 
in several instances apparently in correlation with the 
flesliy nature of the mature plant— so as to contain a 
store of nutriment sufficient for the seedling, and then 



Chap. II. HYPOCOTYLS AND EPICOTYLS. 99 

that one or both cotyledons, from being superfluous, 
decreased in size. It is not surprising that one cotyle- 
don^ alone should sometimes have been thus affected, 
for with certain plants, for instance the cabbage, the 
cotyledons are at first of unequal size, owing apparently 
to the manner in which they are packed within the seed. 
It does not, however, follow from the above connection, 
that whenever a bulb is formed at an early age, one or 
both cotyledons will necessarily become superfluous, and 
consequently more or less rudimentary. Finally, these 
cases offer a good illustration of the principle of compen- 
sation or balancement of growth, or, as Goethe expresses 
it, " in order to spend on one side, Nature is forced to 
economise on the other side." 

Circumnutation and other movements of Hypocotyls 
and Ejncotyls^ whilst still arched and huried heneath 
the ground, and whilst breaking through it. — According 
to the position in which a seed may chance to have been 
buried, the arched hypocotyl or epicotyl will begin to 
protrude in a horizontal, a more or less inclined, or in a 
vertical plane. Except when already standing vertically 
upwards, both legs of the arch are acted on from the 
earliest period by apogeotropism. Consequently they 
both bend upwards, until the arch becomes vertical. 
During the whole of this process, even before the arch has 
broken through the ground, it is continually trying to 
circumnutate to a slight extent; as it likewise does if it 
happens at first to stand vertically up, — all which cases 
have been observed and described, more or less fully, in 
the last chapter. After the arch has grown to some 
height upwards, the basal part ceases to circumnutate, 
whilst the upper part continues to do so. 

That an arched hypocotyl or epicotyl, with the two 
legs fixed in the ground, should be able to circumnutate, 
seemed to us, until we had read Prof. Wiesner's observa- 



100 CIRCUMNUTATING MOVEMENTS OF Chap. II. 

tions, an inexplicable fact. He has shown* in the case 
of certain seedlings, whose tips are bent downwards (or 
which nutate), that whilst the posterior side of the npper 
or dependent portion grows quickest, the anterior and 
opposite side of the basal portion of the same internode 
grows quickest ; these two portions being separated by 
an indifferent zone, where the growth is equal on all 
sides. There may even be more than one indifferent 
zone in the same internode ; and the opposite sides of 
the parts above and below each such zone grow quick- 
est. This peculiar manner of growth is called by Wies- 
ner " undulatory nutation." Circumnutation depends 
on one side of an organ growing quickest (probably 
preceded by increased turgescence), and then another 
side, generally almost the opposite one, growing quickest. 
Now if we look at an arch like this f| and suppose the 
whole of one side — we will say the whole convex side of 
both legs — to increase in length, this would not cause 
the arch to bend to either side. But if the outer side or 
surface of the left leg w^ere to increase in length the arch 
would be pushed over to the right, and this would be 
aided by the inner side of the right leg increasing in 
length. If afterwards the process were reversed, the 
arch would be pushed over to the opposite or left side, 
and so on alternately, — that is, it would circumnutate. 
As an arched hypocotyl, with the two legs fixed in the 
ground, certainly circumuutates, and as it consists of a 
single internode, we may conclude that it grows in the 
manner described by Wiesner. It may be added, that 
the crown of the arch does not grow, or grows very 
slowly, for it does not increase much in breadth, whilst 
the arch itself increases greatly in height. 



*' Die unduliiTiide Nutation der (Vienna), Jan. 17th, 1878. Also 
Intrrnodion,' Akad. der Wissench. published separately see p. 32. 



Chap. IL HYPOCOTYLS, ETC., WHILST AKCHED. IQl 

The circiimnutating movements of arched hypocotyls 
and epicotyls can hardly fail to aid them in breaking 
through the ground, if this be damp and soft ; though 
no doubt their emergence depends mainly on the force 
exerted by their longitudinal growth. Although the 
arch circumnutates only to a slight extent and probably 
with little force, yet it is able to move the soil near the 
surface, though it may not be able to do so at a moderate 
depth. A pot with seeds of Solarium palinacanthum^ 
the tall arched hypocotyls of which had emerged and 
were growing rather slowly, was covered with fine argil- 
laceous sand kept damp, and this at first closely sur- 
rounded the bases of the arches ; but soon a narrow open 
crack was formed round each of them, which could be 
accounted for only by their having pushed away the sand 
on all sides ; for no such cracks surrounded some little 
sticks and pins which had been driven into the sand. It 
has already been stated that the cotyledons of Phalaris 
and Avena, the plumules of Asparagus and the hypo- 
cotyls of Brassica, were likewise able to displace the same 
kind of sand, either whilst simply circumnutating or 
whilst bending towards a lateral light. 

As long as an arched hypocotyl or epicotyl remains 
buried beneath the ground, the two legs cannot separate 
from one another, except to a slight extent from the 
yielding of the soil ; but as soon as the arch rises above 
the ground, or at an earlier period if the pressure of the 
surrounding earth be artificially removed, the arch imme- 
diately begins to straighten itself. This no doubt is due 
to the growth along the wlioh inner surface of both legs 
of the arch ; such growth being checked or prevented, 
as long as the two legs of the arch are firmly pressed 
together. When the earth is removed all round an arch 
and the two legs are tied together at their bases, the 
growth on the under side of the crown causes it after a 



102 EUPTUEE OF THE SEED-COATS. Chap. II. 

time to become much flatter and broader than naturally 
occurs. The straighteuing process consists of a modified 
form of circumnutation, for the lines described during 
this process (as with the hypocotyl of Brassica, and the. 
epicotyls of Vicia and Corylus) were often plainly zigzag 
and sometimes looped. After hypocotyls or epicotyls 
have emerged from the ground, they quickly become 
perfectly straight. No trace is left of their former 
abrupt curvature, excepting in the case of Allium cepa^ 
in which. the cotyledon rarely becomes quite straight, 
owing to the protuberance developed on the crown of 
the arch. 

The increased growth along the inner surface of the 
arch which renders it straight, apparently begins in 
the basal leg or that which is united to the radicle; 
for this leg, as we often observed, is first bowed back- 
wards from the other leg. This movement facilitates 
the withdrawal of the tip of the epicotyl or of the 
cotyledons, as the case may be, from within the seed- 
coats and from the ground. But the cotyledons often 
emerge from the ground still tightly enclosed within 
the seed-coats, which apparently serve to protect them. 
The seed-coats are afterwards ruptured and cast off by 
the swelling of the closely conjoined cotyledons, and not 
by any movement or their separation from one another. 

Nevertheless, in some few cases, especially with the 
Oucurbitaceoe, the seed-coats are ruptured by a curious 
contrivance, described by M. Flahault.* A heel or 
peg is developed on one side of the summit of the 
radicle or base of the hypocotyl ; and this holds down 
the lower half of the seed -coats (the radicle being 
fixed into the ground) whilst the continued growth of 
the arched hypocotyl forces upwards the upper half, and 



Hull. Soc. Hot. do France,' torn. xxiv. 1877, p. 201. 



Chap. II. RUPTURE OF THE SEED-COATS. 



103 



Fig. 62. 




tears asunder the seed-coats at one end, and the cotyle- 
dons are then easily withdrawn. The accompanying 
figure (Fig. 62) will render this description intelligible. 
Forty-one seeds of Cucurlita ovifera were laid on fri- 
able peat and were covered by a layer about an inch in 
thickness, not much pressed down, so that the cotyle- 
dons in being dragged up were 
subjected to very little friction, 
yet forty of them came up naked, 
the seed-coats being left buried 
in the peat. This was certainly 
due to the action of the peg, for 
when it was prevented from act- 
ing, the cotyledons, as we shall 
presently see, were lifted up still 
enclosed in their seed-coats. They 
were, however, cast off in the 
course of two or three days by 
the swelling of the cotyledons. 
Until this occurs light is ex- 
cluded, and the cotyledons can- 
not decompose carbonic acid ; but 
no one probably would have 
thought that the advantage thus 
gained by a little earlier cast- 
ing off of the seed -coats would 
be sufficient to account for the 
development of the peg. Yet, 
according to M. Flahault, seedlings which have been pre- 
vented from casting their seed-coats whilst beneath the 
ground, are inferior to those which have emerged with 
their cotyledons naked and ready to act. 

The peg is developed with extraordinary rapidity; 
for it could only just be distinguished in two seed- 
lings, having radicles -35 inch in length, but after an 



Curcubita ovifera : germi- 
nating seed, showing tlie 
heel or peg projecting 
on one side from summit 
of radicle and holding 
down lower tip of seed- 
coats which have been 
partially ruptured by 
the growth of the arched 
hypocotyl. 



104 RUPTURE OF THE SEED-COATS. Chap. II. 

interval of only 24 hours was well developed in both. 
It is formed, according to Flahault, by the enlargement 
of the layers of the cortical parenchyma at the base of 
the hypocotyl. If, however, we judge by the effects of 
a solution of permanganate of potassium, it is developed 
on the exact line of junction between the hypocotyl and 
radicle ; for the flat lower surface, as well as the edges, 
were coloured brown like the radicle ; whilst the upper 
slightly inclined surface was left uncoloured like the 
hypocotyl, excepting indeed in one out of 33 im- 
mersed seedlings in which a large part of the upper sur- 
face was coloured brown. Secondary roots sometimes 
spring from the lower surface of the peg, which thus 
seems in all respects to partake of the nature of the 
radicle. The peg is always developed on the side which 
becomes concave by the arching of the hypocotyl ; 
and it would be of no service if it were formed on any 
other side. It is also always developed with the flat 
lower side, which, as just stated, forms a part of the 
radicle, at right angles to it, and in a horizontal plane. 
This fact was clearly shown by burying some of the 
thin flat seeds in the same position as in Fig. 62, 
excepting that they were not laid on their flat broad 
sides, but with one edge downwards. Nine seeds 
were thus planted, and the peg was developed in the 
same position, relatively to the radicle, as in the 
figure ; consequently it did not rest on the flat tip 
of the lower half of the seed-coats, but was inserted 
like a wedge between the two tips. As the arched 
hypocotyl grew upwards it tended to draw up the 
whole seed, and the peg necessarily rubbed against 
both tips, but did not hold either down. The result 
was, that the cotyledons of five out of the nine seeds 
tiiiis placed were raised above the ground still enclosed 
within their seed-coats. Four seeds were buried with 



Chap. II. RUPTURE OF THE SEED-COATS. 105 

the end from which the radicle protrudes pointing 
vertically downwards, and owing to the peg being 
always developed in the same position, its apex alone 
came into contact with, and rubbed against the tip on 
one side ; the result was, that the cotyledons of all 
four emerged still within their seed-coats. These cases 
show us how the peg acts in co-ordination with the 
position which the flat, thin, broad seeds would almost 
always occupy when naturally sown. When the tip 
of the lower half of the seed coats was cut off, Flahault 
found (as we did likewise) that the peg could not -act, 
since it had nothing to press on, and the cotyledons 
were raised above the ground with their seed-coats not 
cast off. Lastly, nature shows us the use of the peg ; 
for in the one Cucurbitaceous genus known to us, in 
which the cotyledons are hypogean and do not cast 
their seed -coats, namely, Megarrhiza, there is no 
vestige of a peg. This structure seems to be present 
in most of the other genera in the family, judging from 
Flahault's statements ; we found it well-developed and 
properly acting in Tricliosanthes anguina^ in which we 
hardly expected to find it, as the cotyledons are some- 
what thick and fleshy. Eew cases can be advanced of a 
structure better adapted for a special purpose than the 
present one. 

With Mimosa pudica the radicle protrudes from a 
small hole in the sharp edge of the seed ; and oii its 
summit, where united with the hypocotyl, a transverse 
ridge is developed at an early age, which clearly aids 
in splitting the tough seed-coats ; but it does not aid 
in casting ttiem off, as this is subsequently effected by 
the swelling of the cotyledons after they have been 
raised above the ground. The ridge or heel therefore 
acts rather differently from that of Oucurbita. Its 
lower surface and the edges were coloured brown by 



106 RUPTURE OF THE SEED-COATS. Chap. II. 

the permanganate of potassiam, but not the upper 
surface. It is a singular fact that after the ridge has 
done its work and has escaped from the seed-coats, 
it is developed into a frill all round the summit of the 
radicle.* 

At the base of the enlarged hypocotyl of Ahronia 
iwibeUatcCy where it blends into the radicle, there is a 
projection or heel which varies in shape, but its out- 
line is too angular in our former figure (Fig. 61). The 
radicle first protrudes from a small hole at one end of 
the tough, leathery, winged fruit. At this period the 
upper part of the radicle is packed within the fruit 
parallel to the hypocotyl, and the single cotyledon is 
doubled back parallel to the latter. The swelling of 
these three parts, and especially the rapid development 
of the thick heel between the hypocotyl and radicle 
at the point where they are doubled, ruptures the 
tough fruit at the upper end and allows the arched 
hypocotyl to emerge ; and this seems to be the function 
of the heel. A seed was cut out of the fruit and 
allowed to germinate in damp air, and now a thin 
flat disc was developed all round the base of the 
hypocotyl and grew to an extraordinary breadth, like 
the frill described under Mimosa, but somewhat broader. 
Flahault says that with Mirabilis, a member of the 
same family with Abronia, a heel or collar is developed 
all round the base of the hypocotyl, but more on one 
side than on tlie other ; and that it frees the cotyle- 
dons from their seed - coats. We observed only old 
seeds, and these were ruptured by the absorption of 



* Our attention was called to the junction of the radicle and 

this case by a l)rief statement by hypocotyl. This seed possesses a 

Nobl)e in his ' Handbuch der Sa- very hard and tough coat, and 

nienkunde,' 1H7G. p. 215, where a would be likely to require aid in 

fiKure isalsoKivenofaseedliusof bui-sting and freeing the cotyle- 

Martynia with a heel or ridge at dons. 



Chap. II. CIRCUMNUTATION OF HYPOCOTYLS. IQY 

moisture, independently of any tiid from the heel and 
before the protrusion of the radicle ; but it does not 
follow from our experience that fresh and tough fruits 
would behave in a like manner. 

In concluding this section of the present chapter it 
may be convenient to summarise, under the form of an 
illustration, the usual movements of the hypocotyls 
and epicotyls of seedlings, whilst breaking through the 
ground and immediately afterwards. We may suppose 
a man to be thrown down on his hands and knees, and 
at the same time to one side, by a load of hay falling 
on him. He would first endeavour to get his arched 
back upright, wriggling at the same time in all di- 
rections to free himself a little from the surrounding 
pressure ; and this may represent the combined effects 
of apogeotropism and circumnutation, when a seed is so 
buried that the arched hypocotyl or epicotyl protrudes 
at first in a horizontal or inclined plane. The man, 
still wriggling, would then raise his arched back as 
high as he could ; and this may represent the growth 
and continued circumnutation of an arched hypocotyl 
or epicotyl, before it has reached the surface of the 
ground. As soon as the man felt himself at all free, he 
would raise the upper part of his body, whilst still on 
his knees and still wriggling ; and this may represent 
the bowing backwards of the basal leg of the arch, 
which in most cases aids in the withdrawal of the 
cotyledons from the buried and ruptured seed-coats, 
and the subsequent straiglitening of the whole hypo- 
cotyl or epicotyl — circumnutation still continuing. 

Circumnutation of Hypocotyls and Epicotyls^ tvhcn 
erect. — The hypocotyls, epicotyls, and first shoots of the 
many seedlings observed by us, after they had become 
straight and erect, circumnutated continuously. The 



108 CIRCUMNUTATION OF HYPOCOTYLS. Chap. II. 

diversified figures described by them, often during two 
successive days, have been shown in the woodcuts in 
the last chapter. It should be recollected that the 
dots were joined by straight lines, so that the figures 
are angular ; but if the observations had been made 
every few minutes the lines would have been more 
or less curvilinear, and irregular ellipses or ovals, or 
perhaps occasionally circles, would have been formed. 
The direction of the longer axes of the ellipses made 
during the same day or on successive days generally 
changed completely, so as to stand at right angles to 
one another. The number of irregular ellipses or 
circles made within a given time differs much with 
different species. Thus with Brassica oleracea^ Cerinthe 
7najor, and Cucurhita ovifera abont four such figures 
were completed in 12h.; whereas witb /S'o?«?^w??^ j9«/?'?i<2- 
cantlium and Opuntia basilaris, scarcely more than one. 
The figures likewise diifer greatly in size ; thus they 
were very small and in some degree doubtful in Stapelia, 
and large in Brassica, &c. The ellipses described by 
Lathyrus nissolia and Brassica were narrow, whilst 
those made by the Oak Avere broad. The figures are 
often complicated by small loops and zigzag lines. 

As most seedling plants before the development 
of true leaves are of low, sometimes very low stature, 
the extreme amount of movement from side to side 
of their circumnutating stems was small; that of 
tlic hypocotyl of Githago segetum was about -2 of an 
inch, and that of Cucurbita ovifera about -28. A 
very young shoot of Lathyrus nissolia moved about 
•l-i, that of an American oak -2, that of the common 
nut only -04, and a rather tall shoot of the Asparagus 
•11 of an inch. The extreme amount of movement 
of the slieath-like cotyledon of Plialaris Canariensis 
was -3 of an inch ; but it did not move very quickly, 



Chap. II. CIRCUMNUTATION OF COTYLEDONS. 109 

the tip crossing on one occasion five divisions of the 
micrometer, that is, x^o-th of an inch, in 22 m. 5 s. A 
seedling Nolana prostrata travelled the same distance 
in 10 m. 38 s. Seedling cabbages circamnutated much 
more quickly, for the tip of a cotyledon crossed 
^-^(j-th of an inch on the micrometer in 3 m. 20 s. ; and 
this rapid movement, accompanied by incessant oscil- 
lations, was a wonderful spectacle when beheld under 
the microscope. 

The absence of light, for at least a day, does not 
interfere in the least with the circumnutation of the 
hypocotyls, epicotyls, or young shoots of the various 
dicotyledonous seedlings observed by us ; nor with that 
of the young shoots of some monocotyledons. The 
circumnutation was indeed much plainer in darkness 
than in light, for if the light was at all lateral the 
stem bent towards it in a more or less zigzag course. 

Finally, the hypocotyls of many seedlings are drawn 
during the winter into the ground, or even beneath it 
so that they disappear. This remarkable process, 
which apparently serves for their protection, has 
been fully described by De Vries.* He shows that 
it is effected by the contraction of the parenchyma- 
cells of the root. But the hypocotyl itself in some 
cases contracts greatly, and although at first smooth 
becomes covered with zigzag ridges, as we observed 
with Githago segetum. How much of the drawing 
down and burying of the hypocotyl of Opuntia basilar is 
was due to the contraction of this part and how much 
to that of the radicle, we did not observe. 

Circumnutation of Cotyledons. — With all the dicoty- 
ledonous seedlings described in the last chapter, the 



* ' Bot. Zeitung,' 1879, p. 649. burg,' Jahrg. xvi. p. 16, as quoted 
See also Winkler in 'Verhandl. by Haberlandt, 'Scliutzeinrichtun- 
des Bot, Vereins der P. Branden- gen der Keimpflanze, ' 1877, p. 52. 



110 CIRCUMNUTATION OF COTYLEDONS. Chap. II. 

cotyledons were in constant movement, chiefly in a yer- 
tical plane, and commonly once up and once down in 
the course of the 24 hours. But there were many excep- 
tions to such simplicity of movement ; thus the cotyle- 
dons of Iidomma cmrulea moved 13 times either upwards 
or downwards in the course of 16 h. 18 m. Those of 
Oxalis rosea moved in the same manner 7 times in the 
course of 24 h.; and those of Cassia tora described 5 
irregular ellipses in 9 h. The cotyledons of some indi- 
viduals of Mimosa pndica and of Lotus Jacohmus moved 
only once up and down in 24 h., whilst those of others 
performed within the same period an additional small 
oscillation. Thus with different species, and with dif- 
ferent individuals of the same species, there were many 
gradations from a single diurnal movement to oscilla- 
tions as complex as those of the Ipomoea and Cassia. 
The opposite cotyledons on the same seedling move to a 
certain extent independently of one another. This was 
conspicuous with those of Oxalis sensitiva, in which one 
cotyledon might be seen during the daytime rising up 
until it stood vertically, whilst the opposite one was sink- 
ing down. 

Although the movements of cotyledons were gener- 
ally in nearly the same vertical plane, yet their upward 
and downward courses never exactly coincided ; so that 
ellipses, more or less narrow, were described, and the 
cotyledons may safely be said to have circumnutated. 
Nor could this fact be accounted for by the mere in- 
crease in length of the cotyledons through growth, for 
this by itself would not induce any lateral movement. 
That there was lateral movement in some instances, as 
with the cotyledons of the cabbage, was evident ; for 
these, besides moving up and down, changed their course 
from right to left 12 times in 14 h. 15 m. With Solanum 
hicopcrsicuni tlic cotyledons, after falling in the fore- 



Chap. II. CmCUMNUTATION OF COTYLEDONS. m 

noon, zigzagged from side to side between 12 and 4 p.m., 
and then commenced rising. The cotyledons of Lupi7ius 
luteus are so thick (about -08 of an inch) and fleshy,* 
that they seemed little likely to move, and were there- 
fore observed with especial interest ; they certainly moved 
largely up and down, and as the line traced was zigzag 
there was some lateral movement. The nine cotyledons 
of a seedling Pinus pi7iaster plainly circumnutated ; and 
the figures described approached more nearly to irregu- 
lar circles than to irregular ovals or ellipses. The sheath- 
like cotyledons of the Gramineae circumnutate, that is, 
move to all sides, as plainly as do the hypocotyls or epi- 
cotyls of any dicotyledonous plants. Lastly, the very 
young fronds of a Fern and of a Selaginella circum- 
nutated. 

In a large majority of the cases which were carefully 
observed, the cotyledons sink a little downwards in the 
forenoon, and rise a little in the afternoon or evening. 
They thus stand rather more highly inclined during 
the night than during the mid-day, at which time they 
are expanded almost horizontally. The circumnutating 
movement is thus at least partially periodic, no doubt in 
connection, as we shall hereafter see, with the daily alter- 
nations of light and darkness. The cotyledons of several 
plants move up so much at night as to stand nearly or 
quite vertically ; and in this latter case they come into 
close contact with one another. On the other hand, the 
cotyledons of a few plants sink almost or quite vertically 
down at night ; and in this latter case they clasp the 
upper part of the hypocotyl. In the same genus Oxalis 
the cotyledons of certain species stand vertically up, and 



* The cotyledons, though Ijright ('Die Schutzciiirichtungen,' &c., 

green, resemble to a certain extent 1877, p. 95), on the gradations in 

hypogean ones; see the interest- the Leguminosfe between subacrial 

ing discussion by Haberlandt and subterranean cotyledons. 



112 CIRCUMNUTATION OF COTYLEDONS. Chap. II. 

those of other species vertically down, at night. In all 
such cases the cotyledons may be said to sleep, for they 
act in the same manner as do the leaves of many sleep- 
ing plants. This is a movement for a special purpose, 
and will therefore be considered in a future chapter 
devoted to this subject. 

In order to gain some rude notion of the proportional 
number of cases in which the cotyledons of dicotyledo- 
nous plants (hypogean ones being of course excluded) 
changed their position in a conspicuous manner at night, 
one or more species in several genera were cursorily ob- 
served, besides those described in the last chapter. Alto- 
gether 153 genera, included in as many families as could 
be procured, were thus observed by us. The cotyledons 
were looked at in the middle of the day and again at 
night ; and those were noted as sleeping which stood 
either vertically or at an angle of at least 60° above or be- 
neath the horizon. Of such genera there w^ere 26 ; and 
in 21 of them the cotyledons of some of the species rose, 
and in only 6 sank at night ; and some of these latter 
cases are rather doubtful from causes to be explained in 
the chapter on the sleep of cotyledons. When cotyledons 
which at noon were nearly horizontal, stood at night at 
more than 20° and less than 60° above the horizon, they 
were recorded as " plainly raised ; " and of such genera 
there were 38. We did not meet with any distinct in- 
stances of cotyledons periodically sinking only a few de- 
grees at night, although no doubt such occur. We have 
now accounted for 64 genera out of the 153, and there 
remain 89 in which the cot3'ledons did not change their 
position at night by as much as 20° — that is, in a con- 
spicuous manner which could easily be detected by the 
unaided eye and by memory ; but it must not be inferred 
from this statement that these cotyledons did not move 
at all, for in several cases a rise of a few dearrees was re- 



Chap. II. 



PULVINI OF COTYLEDONS. 



113 



corded, when they were carefully observed. The number 
89 might have been a little increased, for the cotyledons 
remained almost horizontal at night in some species in a 
few genera, for instance, Trifolium and (reranium, which 
are included amongst the sleepers, such genera might 
therefore have been added to the 89. Again, one species 
of Oxalis generally raised its cotyledons at night more 
than 20° and less than 60° above the horizon ; so that 
this genus might have been included under two heads. 
But as several species in the same genus were not often 
observed, such double entries have been avoided. 

In a future chapter it 
will be shown that the leaves 
of many plants which do 
not sleep, rise a few degrees 
in the evening and during 
the early part of the night ; 
and it will be convenient to 
defer until then the consid- 
eration of the periodicity of 
the movements of cotyle- 
dons. 

On the Pulvi7ii or Joints 
of Cotyledons. — With sev- 
eral of the seedlings de- 
scribed in this and the last 
chapter, the summit of the 
petiole is developed into a 
pulvinus, cushion, or joint 
(as this organ has been va- 
riously called), like that 
with which many leaves are 
provided. It consists of a 
mass of small cells usually of a pale colour from the 
absence of chlorophyll, and with its outline more or less 




Oxalis rosea : longitudinal section 
of a pulvinus on the summit 
of the petiole of a cotyledon, 
drawn with the camera lucida, 
magnified 75 times : p, p, pe- 
tiole ; /, fibro-vascular bundle ; 
b, b, commencement of blade of 
cotyledon. 



114 PULVINI OF COTYLEDONS. Chap. II. 

convex, as shown in the annexed figure. In the case 
of Oxalis sensitiva two-thirds of the petiole, and in that 
of Mimosa pudica^ apparently the whole of the short 
sub-petioles of the leaflets have been converted into 
pnlvini. With pulvinated leaves (i.e. those provided 
with a pulvinus) their periodical movements depend, ac- 
cording to Pfeifer,* on the cells of the pulvinus alter- 
nately expanding more quickly on one side than on the 
other ; whereas the similar movements of leaves not pro- 
vided with pnlvini, depend on their growth being alter- 
nately more rapid on one side than on the other. f As 
long as a leaf provided with a pulvinus is young and con- 
tinues to grow, its movement depends on both these causes 
combined ; ^ and if the view now held by many botanists 
be sound, namely, that growth is always preceded by the 
expansion of the growing cells, then the difference be- 
tween the movements induced by the aid of pulvini and 
without such aid, is reduced to the expansion of the cells 
not being followed by growth in the first case, and being 
so followed in the second case. 

Dots were made with Indian ink along the midrib 
of both pulvinated cotyledons of a ratlier old seedling of 
Oxalis Valcliviana; their distances were repeatedly meas- 
ured with an eye-piece micrometer during 8f days, and 
they did not exhibit the least trace of increase. It is 
therefore almost certain that the pulvinus itself was not 
then growing. Nevertheless, during this whole time and 
for ten days afterwards, these cotyledons rose vertically 
every night. In the case of some seedlings raised from 
seeds purchased under the name of Oxalis florihunda, 
the cotyledons continued for a long time to move ver- 
tically down at night, and the movement apparently de- 



'i' ' l)i(> P(>rio(liscb(> T^cwogungon f Ratalin, * Flora,' Oct. 1st, 1873. 

(Icr l.lattorga.U', 18/5. % PfdTer, ibid. p. 5. 



Chap. IT. PULVINI OF COTYLEDONS. 115 

pended exclusively on the piilvini, for their petioles were 
of nearly the same length in young, and in old seedlings 
which had produced true leaves. With some species of 
Cassia, on the other hand, it was obvious without any 
measurement that the pulvinated cotyledons continued 
to increase greatly in length during some weeks ; so that 
here the expansion of the cells of the pulvini and the 
growth of the petiole were probably combined in caus- 
ing their prolonged periodic movements. It was equally 
evident that the cotyledons of many plants, not provided 
with pulvini, increased rapidly in length ; and their 
periodic movements no doubt were exclusively due to 
growth. 

In accordance with the view that the periodic move- 
ments of all cotyledons dej)end primarily on the expan- 
sion of the cells, whether or not followed by growth, we 
can understand the fact that there is but little difference 
in the kind or form of movement in the two sets of cases. 
This may be seen by comparing the diagrams given in 
the last chapter. Thus the movements of the cotyledons 
of Brassica oleracea and of Ipomoea ccerulea^ which are 
not provided with pulvini, are as complex as those of 
Oxalis and Cassia which are thus provided. The pulvi- 
nated cotyledons of some individuals of Mimosa pudica 
and Lotus Jacohmus made only a single oscillation, whilst 
those of other individuals moved twice up and down in 
the course of 24 hours ; so it was occasionally with the 
cotyledons of Cucurbita ovifera^ which are destitute of a 
pulvinus. The movements of pulvinated cotyledons are 
generally larger in extent than those without a pulvinus ; 
nevertheless some of the latter moved through an angle 
of 90''. There is, however, one important difference in 
the two sets of cases ; the nocturnal movements of cotyle- 
dons without pulvini, for instance, those in the Cru- 
ciferae, Cucurbitacege, Githago, and Beta, never last even 



lie PULVINI OF COTYLEDONS. Chap. II. 

for a week, to any conspicuous degree. Pulvinated cotyle- 
dons, on the other hand, continue to rise at night for 
a much longer period, even for more than a month, as 
we shall now show. But the period no doubt depends 
largely on the temperature to which the seedlings are ex- 
posed and their consequent rate of development. 

Oxalis Valdimana. — Some cotyledons which had lately 
opened and were horizontal on March 6th at noon, stood at 
night vertically up; on the 13th the first true leaf was formed, 
and was embraced at night by the cotyledons ; on April 9th, 
after an interval of 35 days, six leaves were developed, and yet 
the cotyledons rose almost vertically at night. The cotyledons 
of another seedling, which when first observed had already pro- 
duced a leaf, stood vertically at night and continued to do so 
for 11 additional days. After 16 days from the first observation 
two leaves were developed, and the cotyledons were still greatly 
raised at right. After 21 days the cotyledons during the day 
were deflected beneath the horizon, but at night were raised 45° 
above it. After 24 days from the first observation (begun after 
a true leaf had been developed) the cotyledons ceased to rise at 
night. 

Oxalis {Biophytum) sensitiva. — The cotyledons of several 
seedHngs, 45 days after their first expansion, stood nearly verti- 
cal at night, and closely embraced either one or two true leaves 
which by this time had been formed. These seedlings had 
been kept in a very warm house, and their development had 
been rapid. 

Oxalis corniculata. — The cotyledons do not stand vertical at 
night, but generally rise to an angle of about 45° above the 
horizon. They continued thus to act for 23 days after their 
first expansion, by which time two leaves had been formed; 
even after 29 days they still rose moderately above their hori- 
zontal or downwardly deflected diurnal position. 

Mimom ]mdiea. — The cotyledons were expanded for the first 
time on Nov. 2nd, and stood vertical at night. On the 15th the 
first leaf was formed, and at night the cotyledons were vertical. 
On the 28 they behaved in the same manner. On Dec. 15th, 
that is after 44 days, the cotyledons were still considerably 



Chap. II. PULVINI OF COTYLEDONS. 117 

raised at night; but those of another seedling, only one day- 
older, were raised very little. 

Mimosa alhida. — A seedling was observed during only 13 
days, by which time a leaf had been formed, and the cotyledons 
were then quite vertical at night. 

Trifolium subterraneum. — A seedling, 8 days old, had its 
cotyledons horizontal at 10.30 a. m. and vertical at 9.15 p.m. 
After an interval of two months, by which time the first and 
second true leaves had been developed, the cotyledons still per- 
formed the same movement. They now increased greatly in 
size, and had become oval ; and their petioles were actually 8 
of an inch in length ! 

Trifolium strictum. — After 17 days the cotyledons still rose 
at night, but were not afterwards observed. 

Lotus Jacotceus. — The cotyledons of some seedlings having 
well- developed leaves rose to an angle of about 45° at night; 
and even after 3 or 4 whorls of leaves had been formed, the 
cotyledons rose at night considerably above their diurnal hori- 
zontal position. 

Cassia mimosoides. — The cotyledons of this Indian species, 
14 days after their first expansion, and when a leaf had been 
formed, stood during the day horizontal, and at night vertical. 

Cassia sp. f (a large S. Brazilian tree raised from seeds sent 
us by F. Miiller). — The cotyledons, after 16 days from their first 
expansion, had increased greatly in size with two leaves just 
formed. They stood horizontally during the day and vertically 
at night, but were not afterwards observed. 

Cassia neglecta (likewise a S. Brazilian species). — A seedling, 
34 days after the first expansion of its cotyledons, was between 
3 and 4 inches in height, with 3 well-developed leaves; and the 
cotyledons, which during the day were nearly horizontal, at night 
stood vertical, closely embracing the young stem. The cotyle- 
dons of another seedling of the same age, 5 inches in height, 
with 4 well-developed leaves, behaved at night in exactly the 
same manner. 

It is known * that there is no difference in structure 
between the upper and lower halves of the pulvini of 

•*" Pfcffer, 'Die Period. Bewegungen,' 1875, p. 157. 



118 PULVmi OF COTYLEDONS. Chap. II. 

leaves, sufficient to account for their upward or down- 
ward movements. In this respect cotyledons offer an 
nnusally good opportunity for comparing the structure 
of the two halves; for the cotyledons of Oxalis Valdi- 
viana rise vertically at night, whilst those of 0. rosea 
sink vertically ; yet when sections of their pulvini were 
made, no clear difference could be detected between the 
corresponding halves of this organ in the two species 
which move so differently. With 0. rosea^ however, 
there w^ere rather more cells in the lower than in the 
upper half, but this was likewise the case in one speci- 
men of 0. Valdiviana. The cotyledons of both species 
(3^ mm. in length) were examined in the morning 
whilst extended horizontally, and the upper surface of 
the pulvinus of 0. rosea was then wrinkled transversely, 
showing that it was in a state of compression, and this 
might have been expected as the cotyledons sink at 
night; with 0. Valdiviana it was the lower surface 
which was wrinkled, and its cotyledons rise at night. 

Trifolium is a natural genus, and the leaves of all 
the species seen by us are pulvinated ; so it is with the 
cotyledons of T. subterraneum and stricturn^ which stand 
vertically at night ; whereas those of T. resupinatum ex- 
hibit not a trace of a pulvinus, nor of any nocturnal 
movement. This was ascertained by measuring the dis- 
tance between the tips of the cotyledons of four seedlings 
at mid-day and at night. In this species, however, as in 
the others, the first-formed leaf, which is simple or not 
trifoliate, rises up and sleeps like the terminal leaflet on 
a mature plant. 

In another natural genus, Oxalis, the cotyledons of 
0. Vahliviana, rosea, floribunda, articiiJata, and sensitiva 
are pulvinated, and all move at night into an upward 
or downward vertical position. In these several species 
the pulvinus is seated close to the blade of the cotyle- 



Chap. II. PULVINI OF COTYLEDONS. 119 

don, as is the usual rule with most plants. Oxalis cor- 
niculata (var. Atro-purptirea) differs in several respects; 
the cotyledons rise at night to a very variable amount, 
rarely more than 45° ; and in one lot of seedlings (pur- 
chased under the name of 0. tropceoloides, but certainly 
belonging to the above variety) they rose only from 5° 
to 15° -above the horizon. The pulvinus is developed im- 
perfectly and to an extremely variable degree, so that 
apparently it is tending towards abortion. No such case 
has hitherto, we believe, been described. It is coloured 
green from its cells containing chlorophyll ; and it is 
seated nearly in the middle of the petiole, instead of at 
the upper end as in all the other species. The nocturnal 
movement is effected partly by its aid, and partly by the 
growth of the upper part of the petiole as in the case of 
plants destitute of a pulvinus. From these sevei'al 
reasons and from our having partially traced the de- 
velopment of the pulvinus from an early age, the case 
seems worth describing in some detail. 

When the cotyledons of 0. corniculata were dissected out of 
a seed from which they would soon have naturally emerged, no 
trace of a pulvinus could be detected ; and all the cells forming 
the short petiole, 7 in number in a longitudinal row, were of 
nearly equal size. In seedlings one or two days old, the pul- 
vinus was so indistinct that we thought at first that it did not 
exist ; but in the middle of the petiole an ill-defined transverse 
zone of cells could be seen, which were much shorter than those 
both above and belo^, although of the same breadth with them. 
They presented the appearance of having been just formed by 
the transverse division of longer cells; and there can be little 
doubt that this had occurred, for the cells in the petiole which 
had been dissected out of the seed averaged in length 7 divisions 
of the micrometer (each division equalling -003 mm.), and were 
a little longer than those forming a well-developed pulvinus, 
which varied between 4 and 6 of these same divisions. After 
a few additional days the ill-defined zone of cells becomes dis- 
9 



120 



PULVINI OF COTYLEDONS. 



Chap. II. 



tinct, and although it does not extend across the whole width 
of the petiole, and although the cells are of a green colour from 
containing chloroijhyll, yet they certainly constitute a pulvinus, 
which, as we shall presently see, acts as one. These small cells 
were arranged in longitudinal rows, and varied from 4 to 7 in 
number; and the cells themselves varied in length in different 
parts of the same pulvinus and in different individuals. In the 
accompanying figures, A and B (Fig. 64), we have views of the 
epidermis * in the middle part of the petioles of two seedlings, 
in which the pulvinus was for this species well developed. They 
offer a striking contrast with the pulvinus of 0. rosea (see former 

Fig. 64. 





Oxalis corniculata : A and B, the almost rudimentary pulvini of the 
cotyledons of two rather old seedlings, viewed as transparent ob- 
jects. Magnified 50 times. 



Fig. 63), or of 0. Valdimana. With the seedlings, falsely called 
0. tropmloides, the cotyledons of which rise very little at night, 
the small cells w^ere still fewer in number and in parts formed 
a single transverse row, and in other parts short longitudinal 
rows of only two or three. Nevertheless they sufficed to attract 
the eye, when the whole petiole was viewed as a transparent 
object beneath the microscope. In these seedlings there could 



•'"Longitudinal sections show sontation of those constituting the 
tint tlie forms of the epidermic pulvinus. 
cells may bo taken :is a fair ropre- 



Chap. II. PULVINI OF COTYLEDONS. 121 

hardly be a doubt that the pulvinus was becoming rudimentary 
and tending to disappear ; and this accounts for its great vari- 
ability in structure and function. 

In the following table some measurements of the cells in 
fairly well-developed pulvini of 0. corniculata are given : — 

Seedling 1 day old, with cotyledon 2 '3 mm. in length. 

Divisions of 
Micrometer. * 

Average length of cells of pulvinus 6 to 7 

Length of longest cell below the pulvinus 13 

Length of longest cell above the pulvinus 20 

Seedling 5 days old, cotyledon 3*1 mm. in length, with the pidvinus 
quite distinct. 

Average length of cells of pulvinus 6 

Length of longest cell below the pulvinus 22 

Length of longest cell above the pulvinus 40 

Seedling 8 days old, cotyledon 5 mm. in length, with a true leaf 
formed hut not yet expanded. 

Average length of cells of pulvinus 9 

Length of longest cell below the pulvinus 44 

Length of longest cell above the pulvinus 70 

Seedling 13 days old, cotyledon 4'5 mm. in length, with a small . 
true leaf fully developed. 

Average length of cells of pulvinus 7 

Length of longest cell below the pulvinus 30 

Length of longest cell above the pulvinus 60 

We here see that the cells of the pulvinus increase but little 
in length with advancing age, in comparison with those of the 
petiole both above and below it ; but they continue to grow in 
width, and keep equal in this respect with the other cells of 
the petiole. The rate of growth, however, varies in all parts 
of the cotyledons, as may be observed in the measurements of 
the 8-days' old seedling. 

The cotyledons of seedlings only a day old rise at night con- 
siderably, sometimes as much as afterwards; but there was 
much variation in this respect. As the pulvinus is so indistinct 
at first, the movement probably does not then depend on the 



*■ Each division equalled '008 mm. 



122 PULVINI OF COTYLEDONS. Chap. II. 

expansion of its cells, but on periodically unequal growth in 
the petiole. By the comparison of seedlings of different known 
ages, it was evident that the chief seat of growth of the petiole 
was in the upper part between the pulvinus and the blade ; and 
this agrees with the fact (shown in the measurements above 
given) that the cells grow to a greater length in the upper than 
in the lower part. With a seedling 11 days old, the nocturnal 
rise was found to depend largely on the action of the pulvinus, 
for the petiole at night was curved upwards at this point ; and 
during the day, whilst the petiole was horizontal, tlie lower 
surface of the pulvinus was wrinkled with the upper surface 
tense. Although the cotyledons at an advanced age do not rise 
at night to a higher inclination than whilst young, yet they have 
to pass through a larger angle (in one instance amounting to 
G3°) to gain their nocturnal position, as they are generally 
deflected beneath the horizon during the day. Even with the 
11-days' old seedling the movement did not depend exclusively 
on the pulvinus, for the blade where joined to the petiole was 
curved upwards, and this must be attributed to unequal growth. 
Therefore the periodic movements of the cotyledons of 0. cornic- 
ulata depend on two distinct but conjoint actions, namely, the 
expansion of the cells of the pulvinus and on the growth of the 
upper part of the petiole, including the base of the blade. 

Lotus Jacobceiis. — The seedlings of this plant present a case 
parallel to that of Oxalis corniculata in some respects, and in 
others unique, as far as we have seen. The cotyledons during 
the first 4 or 5 days of their life do not exhibit any plain noc- 
turnal movement ; but afterwards they stand vertically or 
almost vertically up at night. There is, however, some degree of 
variability in this respect, apparently dependent on the season 
and on the degree to which they have been illuminated during 
the day. With older seedlings, having cotyledons 4 mm. in 
length, which rise considerably at night, there is a well de- 
veloped pulvinus close to the bhide, colourless, and rather nar- 
rower than the rest of the petiole, from which it is abruptly 
separated. It is formed of a mass of small cells of an average 
length of -021 mm. ; whereas the cells in the lower part of the 
])etiole arc about -06 mm., and those in the blade from -034 to 
•04 mm. in length. The epidermic cells in the lower part of 



Chap. II. PULYINI OF COTYLEDONS. 123 

the petiole project conically, and thus differ in shape from those 
over the pulvinus. 

Turning now to very young seedlings, the cotyledons of which 
do not rise at night and are only from 3 to 2| mm. in length, 
their petioles do not exhibit any defined zone of small cells, 
destitute of chlorophyll and differing in shape exteriorly from 
the lower ones. Nevertheless, the cells at the place where a 
pulvinus will afterwards be developed are smaller (being on an 
average "015 mm. in length) than those in the lower parts of 
the same petiole, which gradually become larger in proceeding 
downwards, the largest being -030 mm. in length. At this early 
age the cells of the blade are about '027 mm. in length. We 
thus see that the pulvinus is formed by the cells in the upper- 
most part of the petiole, continuing for only a short time to 
increase in length, then being arrested in their growth, accom- 
panied by the loss of their chlorophyll grains ; whilst the cells 
in the lower part of the petiole continue for a long time to 
increase in length, those of the epidermis becoming more coni- 
cal. The singular fact of the cotyledons of this plant not sleep- 
ing at first is therefore due to the pulvinus not being developed 
at an early age. 

We learn from these two cases of Lotus and Oxalis, 
that the development of a pulvinus follows from the 
growth of the cells over a small defined space of the 
petiole being almost arrested at an early age. With 
Lotus JacohcBus the cells at first increese a little in 
length; in Oxalis corniculata they decrease a little, 
owing to self -division. A mass of such small cells 
forming a pulvinus, might therefore be either acquired 
or lost without any special difficulty,, by different 
species in the same natural genus :' and we know that 
v/ith seedlings of Trifolium, Lotus, and Oxalis some of 
the species have a well -developed pulvinus, and others 
have none, or one in a rudimentary condition. As the 
movements caused by the alternate turgescence of the 
cells in the two halves of a pulvinus, must be largely de- 



124: DISTURBED PBPIODIC MOVEMENTS. Chap. II. 

termined by the extensibility and subsequent contraction 
of their walls, we can perhaps understand why a large 
number of small cells will be more efficient than a small 
number of large cells occupying the same space. As a 
pulvinus is formed by the arrestment of the growth of 
its cells, movements dependent on their action may be 
long-continued without any increase in length of the 
part thus provided ; and such long-continued movements 
seem to be one chief end gained by the development of 
a pulvinus. Long-continued movement would be im- 
possible in any part, without an inordinate increase in 
its length, if the turgescence of the cells was always fol- 
lowed by growth. 

Disturbance of the Periodic Moveme7its of Cotyledons 
hy Light. — The hypocotyls and cotyledons of most seed- 
ling plants are, as is well known, extremely heliotropic ; 
but cotyledons, besides being heliotropic, are affected 
paratonically (to use Sachs' expression) by light ; that 
is, their daily periodic movements are greatly and quick- 
ly disturbed by changes in its intensity or by its absence. 
It is not that they cease to circumnutate in darkness, for 
in all the many cases observed by us they continued to 
do so ; but the normal order of their movements in re- 
lation to the alternations of day and night is much dis- 
turbed or quite annulled. This holds good with species 
the cotyledons of which rise or sink so much at night 
that they may be said to sleep, as well as with others 
which rise onljj^a little. But different species are affect- 
ed in very different degrees by changes in the light. 

For instance, the cotyledons of Beta mdgaris, Solanum lyco- 
persicum, Cerinthe major, and Lupimis luteus, when placed in 
darkness, moved down during the afternoon and early night, 
instead of rising as they would have done if they had been ex- 
posed to the liglit. All the individuals of the Solanum did not 
t)ehavc in the same manner, for the cotyledons of one circum- 



Chap. II. DISTURBED PERIODIC MOVEMENTS. 125 

nutated about the same spot between 3.30 and 10 p.m. The 
cotyledons of a seedling of Oxalis comiculata, which was feebly 
illuminated from above, moved downwards during the first 
morning in the normal manner, but on the second morning it 
moved upwards. The cotyledons of Lotvs Jacobosus were not 
affected by 4 h. of complete darkness, but when placed under 
a double skylight and thus feebly illuminated, they quite lost 
their periodical movements on the third morning. On the other 
hand, the cotyledons of Cvcurbita ovifera moved in the normal 
manner during a whole day in darkness. 

Seedlings of Githago segetum were feebly illuminated from 
above in_ the morning before their cotyledons had expanded, 
and they remained closed for the next 40 h. Other seedlings 
were placed in the dark after their cotyledons had opened in 
the morning and these did not begin to close until about 4 h. 
had elapsed. The cotyledons of Oxalis rosea sank vertically 
downwards after being left for 1 h. 20 m. in darkness; but those 
of some other species of Oxalis were not affected by several 
hours of darkness. The cotyledons of several species of Cassia 
are eminently susceptible to changes in the degree of light to 
which they are exposed : thjis seedlings of an unnamed S. Bra- 
zilian species (a large and beautiful tree) were brought out of 
the hot-house and placed on a table in the middle of a room 
with two north-east and one north-west window, so that they 
were fairly well illuminated, though of course less so than in 
the hot-house, the day being moderately bright ; and after 36 
m. the cotyledons which had been horizontal rose up vertically 
and closed together as when asleep; after thus remaining on the 
table for 1 h. 13 m. they began to open. The cotyledons of 
young seedlings of another Brazilian species and of C. negleda, 
treated in the same manner, behaved similarly, excepting that 
they did not rise up quite so much ; they again became horizon- 
tal after about an hour. 

Here is a more interesting case : seedlings of Cassia tora in 
two pots, which had stood for some time on the table in the 
room just described, had their cotyledons horizontal. One pot 
was now exposed for 3 h. to dull sunshine, and the cotyledons 
remained horizontal ; it was then brought back to the table, and 
after 50 m. the cotyledons had risen 68° above the horizon. 



126 COTYLEDONS SENSITIVE Chap. II. 

The other pot was placed during the same 2 h. behind a screen 
in the room, where the light was very obscure, and the cotyle- 
dons rose 63° above the horizon ; the pot was then replaced on 
the table, and after 50 m. the cotyledons had fallen 33°. These 
two pots with seedlings of the same age stood close together, 
and were exposed to exactly the same amount of light, yet the 
cotyledons in the one pot were rising, whilst those in the other 
pot were at the same time sinking. This fact illustrates in a 
striking manner that their movements are not governed by the 
actual amount, but by a change in the intensity or degree of 
the light. A similar experiment was tried with two sets of 
seedlings, both exposed to a dull light, but different in degree, 
and the result was the same. The movements of the cotyledons 
of this Cassia are, however, determined (as in many other cases) 
largely by habit or inheritance, independently of light; for 
seedlings which had been moderately illuminated during the 
day, were kept all night and on the following morning in com- 
plete darkness; yet the cotyledons were partially open in the 
morning and remained open in the dark for about 6 h. The 
cotyledons in another pot, similarly treated on another occasion, 
were open at 7 a.m. and remained open in the dark for 4 h. 30 
m., after which time they began to close. Yet these same seed- 
lings, when brought in the middle of the day from a moderately 
bright into only a moderately dull light raised, as we have seen, 
their cotyledons high above the horizon. 

Sensitiveness of Cotyledons to contact. — This subject does not 
possess much interest, as it is not known that sensitiveness of 
this kind is of any service to seedling plants. We have observed 
cases in only four genera, though we have vainly observed the 
cotyledons of many others. The genus Cassia seems to be pre- 
eminent in this respect: thus, the cotjdedons of G. tora, when 
extended horizontally, were both lightly tapped with a very thin 
twig for 3 m., and in the course of a few minutes they formed 
together an angle of 90°, so that each had risen 45°. A single 
cotyledon of another seedling was tapped in a like manner for 
1 m., and it rose 27° in 9 m. ; and after eight additional min- 
utes it had risen 10° more; the opposite cotyledon, which was 
not tap])ed, hardly moved at all. The cotyledons in all these 
cases became horizontal again in less than half an hour. The 



Chap. II. TO CONTACT. 127 

pulvinus is the most sensitive part, for on slightly pricking three 
cotyledons with a pin in this part, they rose up vertically; but 
the blade was found also to be sensitive, care having been taken 
that the pulvinus was not touched. Drops of water placed qui- 
etly on these cotyledons produced no effect, but an extremely 
fine stream of water, ejected from a syringe, caused them to 
move upwards. When a pot of seedlings was rapidly hit with 
a stick and thus jarred, the cotyledons rose slightly. When a 
minute drop of nitric acid was placed on both pulvini of a seed- 
ling, the cotyledons rose so quickly that they could easily be 
seen to move, and almost immediately afterwards they began to 
fall; but the pulvini had been killed and became brown. 

The cotyledons of an unnamed species of Cassia (a large tree 
from S. Brazil) rose 31° in the course of 26 m. after the pulvini 
and the blades had both been rubbed during 1 m. with a twig; 
but when the blade alone was similarly rubbed the cotyledons 
rose only 8°. The remarkably long and narrow cotyledons, of a 
third unnamed species from S. Brazil, did not move when their 
blades were rubbed on six occasions with a pointed stick for 
30 s. or for 1 m. ; but when the pulvinus was rubbed and slight- 
ly pricked with a pin, the cotyledons rose in the course of a few 
minutes through an angle of 60°. Several cotyledons of G. ne- 
glecta (likewise from S. Brazil) rose in from 5 m. to 15 m. to 
various angles between 16° and 34°, after being rubbed during 
1 m. with a twig. Their sensitiveness is retained to a somewhat 
advanced age, for the cotyledons of a little plant of 0. neglecta, 
34 days old and bearing three true leaves, rose when lightly 
pinched between the finger and thumb. Some seedlings were 
exposed for 30 m. to a wind (temp. 50° F.) sufficiently strong to 
keep the cotyledons vibrating, but this to our surprise did not 
cause any movement. The cotyledons of four seedlings of the 
Indian G. glauca were either rubbed with a thin twig for 2 m. 
or were lightly pinched : one rose 34"; a second only 0°; a third 
13°; and a fourth 17°. A cotyledon of G. florida similarly 
treated rose 9°; one of G. corynibosa rose 7|^°, and one of the 
very distinct G. mimosoides only 6°. Those of G. 2mhescen8 did 
not appear to be in the least sensitive ; nor were those of G. 
nodosa, but these latter are rather thick and fleshy, and do not 
rise at night or go to sleep. 



128 COTYLEDONS SENSITIVE Chap. II. 

SmitMa sensitiva. — This plant belongs to a distinct sub-order 
of the Leguminosae from Cassia. Both cotyledons of an oldish 
seedling, with the first true leaf partially unfolded, were rubbed 
for 1 m. with a fine twig, and in 5 m. each rose 32° ; they re- 
mained in this position for 15 m., but when looked at again 40 
m. after the rubbing, each had fallen 14°. Both cotyledons of 
another and younger seedling were lightly rubbed in the same 
manner for 1 m., and after an interval of 33 m. each had risen 
30°. They were hardly at all sensitive to a fine jet of water. 
The cotyledons of S. Ffundii, an African water plant, are thick 
and fleshy; they are not sensitive and do not go to sleep. 

Mimosa pudica -dud aldida. — The blades of several cotyledons 
of both these plants were rubbed or slightly scratched with a 
needle during 1 m. or 2 m. ; but they did not move in the least. 
"When, however, the pulvini of six cotyledons of M. pudica were 
thus scratched, two of them were slightly raised. In these two 
cases perhaps the pulvinus was accidentally pricked, for on 
pricking the pulvinus of another cotyledon it rose a little. It 
thus appears that the cotyledons of Mimosa are less sensitive 
than those of the previously mentioned plants.* 

Oxalis sensitiva. — The blades and pulvini of two cotyledons, 
standing horizontally, were rubbed or rather tickled for 30 s. 
with a fine split bristle, and in 10 m. each had risen 48° ; when 
looked at again in 35 m. after being rubbed they had risen 4° 
more ; after 30 additional minutes they were again horizontal. 
On hitting a pot rapidly with a stick for 1 m., the cotyledons 
of two seedlings were considerably raised in the course of 11 m. 
A pot was carried a little distance on a tray and thus jolted; 
and the cotyledons of four seedlings were all raised in 10 m. ; 
after 17 m. one had risen 56°, a second 45°, a third almost 90°, 
and a fourtli 90°. After an additional interval of 40 m. three of 
them had re-expanded to a considerable extent. These obser- 
vations were made before we were aware at what an extraordi- 
narily rapid rate the cotyledons circumnutate, and are therefore 



* The solo notice which we havo Veg.,' 18&2, torn. ii. p. 865\ "les 

met with on the stMisitiveness of cotyledons du Jf. j;«<f/w tendent a 

cotyledons, n>lntes to Mimosa ; for so rapprocher par leurs faces supe- 

Aug. r. Do UuidoUo says C Phys. rieures lorsqu'on les irrite." 



Chap. 11. TO CONTACT. 129 

liable to error. Nevertheless it is extremely improbable that 
the cotyledons in the eight cases given, should all have been 
rising at the time when they were irritated. The cotyledons of 
Oxalis Valdiviana and rosea were rubbed and did not exhibit 
any sensitiveness. 

Finally, there seems to exist some relation between 
the habit of cotyledons rising vertically at night or going 
to sleep, and their sensitiveness, especially that of their 
pulvini, to a touch ; for all the above-named plants sleep 
at night. On the other hand, there are many plants the 
cotyledons of which sleep, and are not in the least sensi- 
tive. As the cotyledons of several species of Cassia are 
easily affected both by slightly diminished light and by 
contact, we thought that these two kinds of sensitiveness 
might be connected ; but this is not necessarily the case, 
for the cotyledons of Oxalis sensitiva did not rise when 
kept on one occasion for 1^ h., and on a second occa- 
sion for nearly 4 h., in a dark closet. Some other cotyle- 
dons, as those of Githago segetum^ are much affected by 
a feeble light, but do not move when scratched by a 
needle. That with the same plant there is some relation 
between the sensitiveness of its cotyledons and leaves 
seems highly probable, for the above described Smithia 
and Oxalis have been called sensitiva, owing to their 
leaves being sensitive ; and though the leaves of the sev- 
eral species of Cassia are not sensitive to a touch, yet if 
a branch be shaken or syringed with water, they partially 
assume their nocturnal dependent position. But the re- 
lation between the sensitiveness to contact of the cotyle- 
dons and of the leaves of the same plant is not very close, 
as may be inferred from the cotyledons of Mimosa pudica 
being only slightly sensitive, whilst the leaves are well 
known to be so in the highest degree. Again, the leaves 
of Neptunia oleracea are very sensitive to a touch, whilst 
the cotyledons do not appear to be so in any degree. 



CHAPTER HI. 

Sensitiveness of the Apex of the Radicle to Contact 
AND to Other Irritants. 

Manner in whicli radicles bend when they encounter an obstacle in 
the soil — Vicia faba, tips of radicles highly sensitive to contact 
and other irritants — Eflfects of too high a temperature — Power of 
discriminating between objects attached on opposite sides — Tips of 
secondary radicles sensitive — Pisum, tips of radicles sensitive — 
Effects of such sensitiveness in overcoming geotropism— Secondary 
radicles — Phaseolus, tips of radicles hardly sensitive to contact 
but highly sensitive to caustic and to the removal of a slice — Tro- 
pseolum — Gossypium — Cucurbita — Eaphanus — Ji]sculus,tip not sen- 
sitive to slight contact, highly sensitive to caustic — Quercus, tip 
highly sensitive to contact — Power of discrimination — Zea tip 
highly sensitive, secondary radicles — Sensitiveness of radicles to 
moist air— Summary of chapter. 

Ik order to see how the radicles of seedlings would 
pass over stones, roots and other obstacles which they 
must incessantly encounter in the soil, germinating beans 
( Vicia faba) were so placed that the tips of the radicles 
came into contact, almost rectangularly or at a high 
angle, with underlying plates of glass. In other cases 
the beans were turned about whilst their radicles were 
growing, so that tliey descended nearly vertically on their 
own smooth, almost flat, broad upper surfaces. The 
delicate root-cap, when it first touched any directly 
opposing surface, was a little flattened transversely ; the 
flattening soon became oblique, and in a few hours quite 
disappeared, the apex now pointing at right angles, or at 
nearly right angles, to its former course. The radicle 
then seemed to glide in its new direction over the surface 



Chap. III. SENSITIVENESS OP EADICLES. 131 

which had opposed it, pressing on it with very little force. 
How far such abrupt changes in its former course are 
aided by the circumnutation of the tip must be left 
doubtful. Thin slips of wood were cemented on more or 
less steeply inclined glass-plates, at right angles to the 
radicles which were gliding down them. Straight lines 
had been painted along the growing terminal part of 
some of these radicles, before they met the opposing slip 
of wood ; and the lines became sensibly curved in 2 h. 
after the apex had come into contact with the slips. In 
one case of a radicle, which was growing rather slowly, 
the root-cap, after encountering a rough slip of wood at 
right angles, was at first slightly flattened transversely : 
after an interval of 2 h. 30 m. the flattening became 
oblique ; and after an additional 3 hours the flattening 
had wholly disappeared, and the apex now pointed at 
right angles to its former course. It then continued to 
grow in its new direction alongside the slip of wood, 
until it came to the end of it, round which it bent 
rectangularly. Soon afterwards when coming to the 
edge of the plate of glass, it was again bent at a large 
angle, and descended perpendicularly into the damp 
sand. 

When, as in the above cases, radicles encountered an 
obstacle at right angles to their course, the terminal 
growing part became curved for a length of between '3 
and -4 of an inch (8-10 mm.), measured from the apex. 
This was well' shown by. the black lines which had been 
previously painted on them. The first and most obvious 
explanation of the curvature is, that it results merely 
from the mechanical resistance to the growth of the 
radicle in its original direction. Nevertheless, this expla- 
nation did not seem to us satisfactory. The radicles did 
not present the appearance of having been subjected to 
a sufficient pressure to account for their curvature ; and 



132 SENSITIVENESS OF HADICLES. Chap. III. 

Sachs has shown * that the growing part is more rigid 
than the part immediately above which has ceased to 
grow, so that the latter might have been expected to 
yield and become curved as soon as the apex encountered 
an unyielding object ; whereas it was the stiS growing 
part which became curved. Moreover, an object which 
yields with the greatest ease will deflect a radicle : thus, 
as we have seen, when the apex of the radicle of the bean 
encountered the polished surface of extremely thin tin- 
foil laid on soft sand, no impression was left on it, yet the 
radicle became deflected at right angles. A second 
explanation occurred to us, namely, that even the gen- 
tlest pressure might check the growth of the apex, and 
in this case growth could continue only on one side, 
and thus the radicle would assume a rectangular form ; 
but this view leaves wholly unexplained the curvature of 
the upper part, extending for a length of 8-10 mm. 

We were therefore led to suspect that the apex was 
sensitive to contact, and that an effect was transmitted 
from it to the upper part of the radicle, which was thus 
excited to bend away from the touching object. As a 
little loop of fine thread hung on a tendril or on the 
petiole of a leaf-climbing plant, causes it to bend, we 
thought that any small hard object affixed to the tip of a 
radicle, freely suspended and growing in damp air, might 
cause it to bend, if it were sensitive, and yet would not 
offer any mechanical resistance to its growth. Full de- 
tails will be given of the experiments which were tried, 
as the result proved remarkable. The fact of the apex 
of a radicle being sensitive to contact has never been 
observed, though, as we shall hereafter see, Sachs discov- 
ered that the radicle a little above the apex is sensitive, 
and -bends like a tendril towards the touching object. 



' Arbeiten Bot. Inst. Wiirzbnrg,' Heft iii. 1873, p. 398. 



Chap. III. SENSITIVENESS OF EADICLES. 133 

But when one side of the apex is pressed by any object, 
the growing part bends mvay from the object ; and this 
seems a beautiful adaptation for avoiding obstacles in 
the soil, and, as we shall see, for following the lines of 
least resistance. Many organs, when touched, bend in 
one fixed direction, such as the stamens of Berberis, the 
lobes of Diongea, &c. ; and many organs, such as tendrils, 
whether modified leaves or flower-peduncles, and some 
few stems, bend towards a touching object ; but no case, 
we believe, is known of an organ bending away from a 
touching object. 

Sensitiveness of tlie Apex of the Radicle of Vicia 
faha. — Common beans, after being soaked in water for 
24 h., were pinned with the hilum downwards (in the 
manner followed by Sachs), inside the cork lids of glass- 
vessels, which were half filled with water ; the sides and 
the cork were well moistened, and light was excluded. 
As soon as the beans had protruded radicles, some to a 
length of less than a tenth of an inch, and others to a 
length of several tenths, little squares or oblongs of card 
were affixed to the short sloping sides of their conical 
tips. The squares therefore adhered obliquely with ref- 
erence to the longitudinal axis of the radicle ; and this 
is a very necessary precaution, for if the bits of card ac- 
cidentally became displaced, or were drawn by the viscid 
matter employed, so as to adhere parallel to the side of 
the radicle, although only a little way above the conical 
apex, the radicle did not bend in the peculiar manner 
which we are here considering. Squares of about the 
^V^h of an inch (i e. about 1\ mm.), or oblong bits of 
nearly the same size, were found to be the most conven- 
ient and effective. We employed at first ordinary thin 
card, such as visiting cards, or bits of very thin glass, 
and various other objects; but afterwards sand-paper 



134 SENSITIVENESS OP THE APEX Chap. III. 

was chiefly employed, for it was almost as stiff as thin 
card, and the roughened surface favoured its adhesion. 
At first we generally used very thick gum- water; and 
this of course, under the circumstances, never dried in 
the least ; on the contrary, it sometimes seemed to ab- 
sorb vapour, so that the bits of card became separated 
by a layer of fluid from the tip. When there was no 
such absorption and the card was not displaced, it acted 
well and caused the radicle to bend to the opposite side. 
I should state that thick gum-water by itself induces 
no action. In most cases the bits of card were touched 
with an extremely small quantity of a solution of shellac 
in spirits of wine, which had been left to evaporate 
until it was thick ; it then set hard in a few seconds, and 
fixed the bits of card well. When small drops of the 
shellac were placed on the tips w^ithout any card, they 
set into hard little beads, and these acted like any other 
hard object, causing the radicles to bend to the opposite 
side. Even extremely minute beads of the shellac occa- 
sionally acted in a slight degree, as will hereafter be de- 
scribed. But that it was the cards which chiefly acted 
in our many trials, was proved by coating one side of the 
tip with a little bit of goldbeaters' skin (which by itself 
hardly acts), and then fixing a bit of card to the skin 
with shellac which never came into contact with the 
radicle : nevertheless the radicle bent away from the 
attached card in the ordinary manner. 

Some preliminary trials were made, presently to bo 
described, by which the proper temperature was deter 
mined, and then the following experiments were made. 
It should be premised that the beans were always fixed 
to the cork-lids, for the convenience of manipulation, 
with the edge from which the radicle and plumule 
protrudes, outwards; and it must be remembered that 
owing to what we have called Sachs' curvature, the 



Chap. III. OF .THE RADICLE OF THE BEAN. 



135 



radicles, instead of growing perpendicularly downwards, 
often bend somewhat, even as much as about 45° in- 
wards, or under the suspended bean. Therefore when a 
square of card was fixed to the apex in front, the bowing 




Vieia faba : A, radicle beginning to bend from the attached little 
square of card ; B, bent at a rectangle ; C, bent into a circle or 
loop, with the tip beginning to bend downwards through the 
action of geotropism. 

induced by it coincided with Sachs' curvature, and could 
be distinguished from it only by being more strongly 
pronounced or by occurring more quickly. To avoid 
this source of doubt, the squares were fixed either be- 
hind, causing a curvature in direct opposition to that of 
Sachs', or more comm-only to the right or left sides. 
For the sake of brevity, we will speak of the bits of card, 

&c., as fixed in front, or behind, or laterally. As the 
10 



136 SENSITIVENESS OF THE APEX Chap. III. 

chief curvature of the radicle is at a little distance from 
the apex, and as the extreme terminal and basal portions 
are nearly straight, it is possible to estimate in a rough 
manner the amount of curvature by an angle ; and when 
it is said that the radicle became deflected at any angle 
from the perpendicular, this implies that the apex was 
turned upwards by so many degrees from the downward 
direction which it would naturally have followed, and to 
the side opposite to that to which the card was affixed. 
That the reader may have a clear idea of the kind of 
movement excited by the bits of attached card, we ap- 
pend here accurate sketches of three germinating beans 
thus treated, and selected out of several specimens to 
show the gradations in the degrees of curvature. We 
will now give in detail a series of experiments, and after- 
wards a summary of the results. 

In the first 12 trials, little squares or oblongs of sanded 
card, 1-8 ram. in length, and 1-5 or only 0-9 mm. in breadth 
(i.e. -071 of an inch in length and -059 or -035 of an inch in 
breadth) were fixed with shellac to the tips of the radicles. In 
the subsequent trials the little squares were only occasionally 
measured, but were of about the same size. 

(1.) A young radicle, 4 mm. in length, had a card fixed 
behind : after 9 h. deflected in the plane in which the bean is 
flattened, 50° from the perpendicular and from the card, and in 
opposition to Sachs' curvature: no change next morning, 23 h. 
from the time of attachment. 

(2.) Radicle 5*5 mm. in length, card fixed behind : after 9 h. 
deflected in the plane of tlie bean 20° from the perpendicular 
and from the card, and in opposition to Sachs' curvature : after 
23 h. no change. 

(3.) Radicle 11 mm. in length, card fixed behind: after 9 h. 
deflected in the i)lane of the bean 40° from the perpendicular 
and from the card, and in opposition to Sachs' curvature. The 
tip of the radicle more curved than the upper part, but in the 
same plane. After 23 h. the extreme tip was slightly bent to- 



Chap. III. OF THE RADICLE OF THE BEAN". 137 

wards the card ; the general curvature of the radicle remaining 
the same. 

(4.) Radicle 9 mm. long, card fixed behind and a little 
laterally: after 9 h. deflected in the plane of the bean only 
about 7° or 8° from the perpendicular and from the card, in 
opposition to Sachs' curvature. There was in addition a slight 
lateral curvature directed partly from the card. After 23 h. no 
change. 

(5.) Radicle 8 mm. long, card affixed almost laterally : after 
9 h. deflected 30° from the perpendicular, in the plane of the 
bean and in opposition to Sachs' curvature ; also deflected in a 
plane at right angles to the above one, 20° from the perpen- 
dicular: after 23 h. no change, 

(6.) Radicle 9 mm. long, card affixed in front: after 9 h. de- 
flected in the plane of the bean about 40° from the vertical, 
away from the card and in the direction of Sachs' curvature. 
Here therefore we have no evidence of the card being the cause 
of the deflection, except that a radicle never moves spon- 
taneously, so far as we have seen, as much as 40° in the course 
of 9 h. After 23 h. no change. 

(7.) Radicle 7 mm. long, card affixed to the back: after 9 h. 
the terminal part of the radicle deflected in the plane of the 
bean 20° from the vertical, away from the card and in opposi- 
tion to Sachs' curvature. After 22 h. 30 m. this part of the 
radicle had become straight. 

(8.) Radicle 12 mm. long, card affixed almost laterally: after 
9 h. deflected laterally in a plane at right angles to that of the 
bean between 40° and 50° from the vertical and from the card. 
In the plane of the bean itself the deflection amounted to 8° or 
9° from the vertical and from the card, in opposition to Sachs' 
curvature. After 22 h. 30 m. the extreme tip liad become 
slightly curved towards the card. 

(9.) Card fixed laterally: after 11 h. 30 m. no effect, the 
radicle being still almost vertical. 

(10.) Card fixed almost laterally: after 11 h. 30 m. deflected 
90* from the vertical and from the card, in a plane intermediate 
between that of the bean itself and one at right angles to it. 
Radicle consequently partially deflected from Sachs' curvature. 

(11.) Tip of radicle protected with goldbeaters' skin, with 



138 SENSITIVENESS OF THE APEX Chap. III. 

a square of card of the usual dimensions affixed with shellac ; 
after 11 h. greatly deflected in the plane of the bean, in the 
direction of Sachs' curvature, but to a much greater degree and 
in less time than ever occurs spontaneously. 

(12.) Tip of radicle protected as in last case: after 11 h. no 
effect, but after 24 h. 40 m. radicle clearly deflected from the 
card. This slow action was probably due to a portion of the 
goldbeaters' skin having curled round and lightly touched the 
opposite side of the tip and thus irritated it. 

(13.) A radicle of considerable length had a small square of 
card fixed with shellac to its apex laterally: after only 7 h. 15 m. 
a length of -4 of an inch from the apex, measured along the 
middle, was considerably curved from the side bearing the card. 

(14.) Case like the last in all respects, except that a length 
of only -25 of an inch of the radicle was thus deflected. 

(15.) A small square of card fixed with shellac to the apex 
of a young radicle ; after 9 h. 15 m. deflected through 90° from 
the perpendicular and from the card. After 24 h. deflection 
much decreased, and after an additional day, reduced to 23° 
from the perpendicular. 

(16.) Square of card fixed with shellac behind the apex of a 
radicle, which from its position having been changed during- 
growth had become very crooked; but the terminal portion 
was straight, and this became deflected to about 45° from 
the perpendicular and from the card, in opposition to Sachs' 
curvature. 

(17.) Square of card affixed with shellac: after 8 h. radicle 
curved at right angles from the perpendicular and from the 
card. After 15 additional hours curvature much decreased. 

(18.) Square of card afiixcd with shellac : after 8 h. no effect, 
after 23 h. 3 m. from time of affixing, radicle much curved from 
the square. 

(19.) Square of card affixed with shellac: after 24 h. no 
effect, but the radicle had not grown well and seemed sickly. 

(20.) Square of card affixed with shellac: after 24 h. no 
effect. 

(21, 22.) Squares of card aflixed with shellac; after 24 h. 
radicles of both curved at about 45° from the perpendicular and 
from the cards. 



Chap. III. OF THE RADICLE OP THE BEAN. 139 

(23.) Square of card fixed with shellac to young radicle: 
after 9 h. very slightly curved from the card; after 24 h. tip 
curved towards card. Refixed new square laterally, after 9 h. 
distinctly curved from the card, and after 24 h. curved at right 
angles from the perpendicular and from the card. 

(24.) A rather large oblong piece of card fixed with shellac 
to apex : after 24 h. no effect, but the card was found not to be 
touching the apex. A small square was now refixed with 
shellac; after 16 h. slight deflection from the perpendicular and 
from the card. After an additional day the radicle became 
almost straight. 

(25.) Square of card fixed laterally to apex of young radicle: 
after 9 h. deflection from the perpendicular considerable ; after 
24 h, deflection reduced. Refixed a fresh square with shellac: 
after 24 h. deflection about 40" from the perpendicular and 
from the card. 

(26.) A very small square of card fixed with shellac to apex 
of young radicle: after 9 h. the deflection. from the perpendicu- 
lar and from the card amounted to nearly a right angle ; after 
24 h. deflection much reduced; after an additional 24 h. radicle 
almost straight. 

(27.) Square of card fixed wath shellac to apex of young 
radicle: after 9 h. deflection from the card and from the per- 
pendicular a right angle; next morning quite straight. Re- 
fixed a square laterally with shellac; after 9 h. a little deflec- 
tion, which after 24 h. increased to nearly 20° from the perpen- 
dicular and from the card. 

(28.) Square of card fixed with shellac; after 9 h. some 
deflection; next morning the card dropped off; refixed it with 
shellac; it again became loose and w^as refixed; and now on the 
third trial the radicle was deflected after 14 h. at right angles 
from the card. 

(29.) A small square of card was first fixed w^th thick gum- 
water to the apex. It produced a slight effect but soon fell 
off. A similar square was now affixed laterally with shellac: 
after 9 h. the radicle was deflected nearly 45° from the perpen- 
dicular and from the card. After 36 additional hours angle of 
deflection reduced to about 30°. 

(30.) A very small piece, less than ^^^^^ ^^ ^^ i^^^^ square. 



140 SENSITIVENESS OF THE APEX Chap. III. 

of thin tin-foil fixed with shellac to the apex of a young radicle ; 
after 24 h. no effect. Tin-foil removed, and a small square of 
sanded card fixed with shellac; after 9 h. deflection at nearly 
right angles from the perpendicular and from the card. Next 
morning deflection reduced to about 40° from the perpen- 
dicular. 

(31.) A splinter of thin glass gummed to apex, after 9 h. no 
effect, but it was then found not to be touching the apex of the 
radicle. Next morning a square of card was fixed with shellac 
to it, and after 9 h. radicle greatly deflected from the card. 
After two additional days the deflection had decreased and was 
only 35° from the perpendicular. 

(33.) Small square of sanded card, attached with thick gum- 
water laterally to the apex of a long straight radicle : after 9 h. 
greatly deflected from the perpendicular and from the card. 
Curvature extended for a length of -22 of an inch from the apex. 
After 3 additional hours terminal portion deflected at right 
angles from the perpendicular. Next morning the curved por- 
tion was "36 in length. 

(33.) Square of card gummed to apex: after 15 h. deflected 
at nearly 90° from the perpendicular and from the card. 

(34.) Small oblong of sanded card gummed to apex: after 
15 h. deflected 90° from the perpendicular and from the card: 
in the course of the three following days the terminal portion 
became much contorted and ultimately coiled into a helix. 

(35.) Square of card gummed to apex: after 9 h. deflected 
from card: after 24 h. from time of attachment greatly deflected 
obliquely and partly in opposition to Sachs' curvature. 

(3G.) Small pieces of card, rather less than ^^gth of an inch 
square, gummed to apex : in 9 h. considerably deflected from 
card and in opposition to Sachs' curvature; after 24 h. greatly 
deflected in tlie same direction. After an additional day the 
extreme tip was curved towards the card. 

(37.) Square of card, gummed to apex in front, caused after 
8 h. 30 m. hardly any effect; refixed fresh square laterally, after 
15 h. deflected almost 90° from the perpendicular and from the 
card. After 2 additional days deflection much reduced. 

(38.) Square of card gummed to apex: after 9 h. much de- 
flection, which after 24 li. from time of fixing increased to nearly 



I 



Chap. III. OF THE RADICLE OF THE BEAN. 141 

90°. After an additional day terminal portion was curled into 
a looj), and on the following day into a helix. 

(39.) Small oblong piece of card gummed to apex, nearly in 
front, but a little to one side; in 9 h. slightly deflected in the 
direction of Sachs' curvature, but rather obliquely, and to side 
opposite to card. Xext day more curved in the same direction, 
and after 2 additional days coiled into a ring. 

(40.) Square of card gummed to apex: after 9 h. slightly 
curved from card ; next morning radical straight, and apex had 
grown beyond the card. Refixed another square laterally with 
shellac ; in 9 h. deflected laterally, but also in the direction of 
Sachs' curvature. After 2 additional days' curvature consider- 
ably increased in the same direction. 

(41.) Little square of tin-foil fixed with gum to one side 
of apex of a young and short radicle : after 15 h. no effect, but 
tin-foil had become displaced. A little square of card was now 
gummed to one side of apex, which after 8 h. 40 m. was slightly 
deflected; in 24 h. from the time of attachment deflected at 90° 
from the perpendicular and from the card ; after 9 additional 
hours became hooked, with the apex pointing to the zenith. 
In 3 days from the time of attachment the terminal portion of 
the radicle formed a ring or circle. 

(42.) A little square of thick letter-paper gummed to the 
apex of a radicle, which after 9 h. was deflected from it. In 
24 h. from time when the paper was affixed the deflection much 
increased, and after 2 additional days it amounted to 50° from 
the perpendicular and from the paper. 

(43.) A narrow chip of a quill was flxed with shellac to the 
apex of a radicle. After 9 h. no effect; after 24 h. moderate 
deflection, but now the quill had ceased to touch the apex. 
Removed quill and gummed a little square of card to apex, 
which after 8 h. caused slight deflection. On the fourth day 
from the first attachment of any object, the extreme tip was 
curved towards the card. 

(44.) A rather long and narrow splinter of extremely thin 
glass, fixed with shellac to apex, it caused in 9 h. slight deflec- 
tion, which disappeared in 24 h. ; the splinter was then found 
not touching the apex. It was twice refixed, with nearly simi- 
lar results, that is, it caused slight deflection, which soon dis- 



142 SENSITIVENESS OF THE APEX Chap. III. 

appeared. On the fourth day from the time of first attachment 
tJie tip was bent towards tlie splinter. 

From these experiments it is clear that the apex of 
the radicle of the bean is sensitive to contact, and that it 
causes the upper part to bend away from the touching 
object. But before giving a summary of the results, it 
will be convenient briefly to give a few other observa- 
tions. Bits of very thin glass and little squares of com- 
mon card were aflixed with thick gum-water to the tips 
of the radicles of seven beans, as a preliminary trial. 
Six of these were plainly acted on, and in two cases the 
radicles became coiled up into complete loops. One radi- 
cle was curved into a semicircle in so short a period as 
6 h. 10 m. The seventh radicle which was not affected 
was apparently sickly, as it became brown on the follow- 
ing day ; so that it formed no real exception. Some of 
these trials were made in the early spring during cold 
weather in a sitting-room, and others in a greenhouse, 
but the temperature was not recorded. These six strik- 
ing cases almost convinced us that the apex was sensitive, 
but of course we determined to make many more trials. 
As we had noticed that the radicles grew much more 
quickly when subjected to considerable heat, and as we 
imagined tliat heat would increase their sensitiveness, ves- 
sels with germinating beans suspended in damp air were 
])laced on a cliimiiey-piece, where they were subjected dur- 
ing the greater ])art of the day to a temperature of be- 
tween 09° and 72° F. ; some, however, were placed in the 
liot-house where the temperature was rather higher. 
Above two dozen boans were thus tried ; and when a square 
of glass or card did not act, it was removed, and a fresh 
one aflixed, this being often done thrice to the same 
radicle. Therefore between five and six dozen trials were 
altogether made. I^ut there was moderately distinct 



Chap. III. OF THE EADICLB OF THE BEAN. 143 

deflection from the perpendicular and from the attached 
object in only one radicle out of this large number of 
cases. In five other cases there was very slight and 
doubtful deflection. We were astonished at this result, 
and concluded that we had made some inexplicable mis- 
take in the first six experiments. But before finally relin- 
quishing the subject, we resolved to make one other trial, 
for it occurred to us that sensitiveness is easily affected 
by external conditions, and that radicles growing natu- 
rally in the earth in the early spring would not be sub- 
jected to a temperature nearly so high as 70° F. We 
therefore allowed the radicles of 12 beans to grow at a 
temperature of between 55° and 60° F. The result was 
that in every one of these cases (included in the above- 
described experiments) the radicle was deflected in the 
course of a few hours from the attached object. All the 
above recorded successful trials, and some others pres- 
ently to be given, were made in a sitting-room at the 
temperatures just specified. It therefore appears that a 
temperature of about, or rather above, 70° F. destroys 
the sensitiveness of the radicles, either directly, or indi- 
rectly through abnormally accelerated growth ; and this 
curious fact probably explains why Sachs, who expressly 
states that his beans were kept at a high temperature, 
failed to detect the sensitiveness of the apex of the radicle. 
But other causes interfere with the sensibility. 
Eighteen radicles were tried with little squares of 
sanded card, some affixed with shellac and some with 
gum-water, during the few last days of 1878, and few 
first days of the next year. They were kept in a room at 
the proper temperature during tlie day, but were prob- 
ably too cold at night, as there was a hard frost at the 
time. The radicles looked healthy, but grew very slowly. 
The result was that only 6 out of the 18 were deflected 
from the attached cards, and this only to a slight degree 



144 SENSITIVENESS OF THE APEX Chap. III. 

and at a very slow rate. These radicles therefore pre- 
sented a striking contrast with the 44 above described. 
On March 6th and 7th, when the temperature of the 
room varied between 53° and 59° F., eleven germinating 
beans were tried in the same manner, and now every one 
of the radicles became aurved away from the cards, 
though one was only slightly deflected. Some horticul- 
turists believe that certain kinds of seeds will not germi- 
nate properly in the middle of the winter, although kept 
at a right temperature. If there really is any proper 
period for the germination of the bean, the feeble degree 
of sensibility of the above radicles may have resulted 
from the trial having been made in the middle of the 
winter, and not simply from the nights being too cold. 
Lastly, the radicles of four beans, which, from some in- 
nate cause germinated later than all the others of the 
same lot, and which grew slowly though appearing 
healthy, were similarly tried, and even after 24 h. they 
were hardly at all deflected from the attached cards. We 
may therefore infer that any cause which renders the 
growth of the radicles either slower or more rapid than 
the normal rate, lessens or annuls the sensibility of their 
tips to contact. It deserves particular attention that 
when the attached objects failed to act, there was no 
bending of any kind, excepting Sachs' curvature. The 
force of our evidence would have been greatly weakened 
if occasionally, though rarely, the radicles had become 
curved in any direction independently of the attached 
objects. In the foregoing numbered paragraphs, how- 
over, it may be observed that the extreme tip sometimes 
becomes, after a considerable interval of time, abruptly 
curved towards the bit of card ; but this is a totally dis- 
tinct phenomenon, as will presently be explained. 

Summary of the ResuUs of the foregoing Experi- 
ments on tht Radides of Vicia faba.—AMogether little 



Chap. III. OF THE RADICLE OF THE BEAN. I45 

squares (about ^V^h of an inch), generally of sanded 
paper as stiff as thin card (between -15 and -20 mm. in 
thickness), sometimes of ordinary card, or little frag- 
ments of very thin glass, &c., were affixed at different 
times to one side of the conical tips of 55 radicles. The 
11 last-mentioned cases, but not the preliminary ones, 
are here included. The squares, &c., were most com- 
monly affixed with shellac, but in 19 cases with thick 
gum-water. When the latter was used, the squares were 
sometimes found, as previously stated, to be separated 
from the apex by a layer of thick fluid, so that there 
was no contact, and consequently no bending of the 
radicles ; and such few cases were not recorded. Bat 
in every instance in which shellac was employed, unless 
the square fell off very soon, the result was recorded. 
In several instances when the squares became displaced, 
so as to stand parallel to the radicle, or were separated 
by fluid from the apex, or soon fell off, fresh squares 
were attached, and these cases (described under the 
numbered paragraphs) are here included. Out of 55 
radicles experimented on under the proper temperature, 
52 became bent, generally to a considerable extent from 
the perpendicular, and away from the side to which the 
object was attached. Of the three failures, one can be 
accounted for, as the radicle became sickly on the fol- 
lowing day ; and a second was observed only during 11 
h. 30. As in several cases the terminal growing part of 
the radicle continued for some time to bend from the 
attached object, it formed itself into a hook, with the 
apex pointing to the zenith, or even into a ring, and oc- 
casionally into a spire or helix. It is remarkable that 
these latter cases occurred more frequently when objects 
were attached with thick gum-water, which never be- 
came dry, than when shellac was employed. The curva- 
ture was often well-marked in from 7 h. to 11 h. ; and 



14G SENSITIVENESS OF THE APEX Cuap. III. 

ill one instance a semicircle was formed in 6 h. 10 m. 
from the time of attachment. But in order to see the 
phenomenon as well displayed as in the above described 
cases, it is indispensable that the bits of card, &c., should 
be made to adhere closely to one side of the conical 
apex ; that healthy radicles should be selected and kept 
at not too high or too low a temperature, and apparently 
that the trials should not be made in the middle of the 
winter. 

In ten instances, radicles which had curved away 
from a square of card or other object attached to their 
tips, straightened themselves to a certain extent, or even 
completely, in the course of from one to two days from 
the time of attachment. This was more especially apt 
to occur when the curvature was slight. But in one in- 
stance (No. 27) a radicle which in 9 h. had been de- 
flected about 90° from the perpendicular, became quite 
straight in 24 h. from the period of attachment. With 
No. 26, the radicle was almost straight in 48 h. We at 
first attributed the straightening process to the radicles 
becoming accustomed to a slight stimulus, in the same 
manner as a tendril or sensitive petiole becomes accus- 
tomed to a very light loop of thread, and unbends itself 
though the loop remains still suspended ; but Sachs 
states * that radicles of the bean placed horizontally in 
damp air after curving downwards through geotropism, 
straighten themselves a little by growth along their 
lower or concave sides. Why this should occur is not 
clear ; but perhaps it likewise occurred in the above ten 
cases. There is another occasional movement which 
must not be passed over : the tip of the radicle, for 
a length of from 2 to 3 mm., was found in six in- 
stances, after an interval of about 24 or more hours, bent 



•••■ ' Arl)inten Hot. Instit.. Wiirzl)urg,' Heft iii. p. 456. 



Chap. III. OF THE RADICLE OF THE BEAN. 14Y 

towards the bit of still attached card, — that is, in a di- 
rection exactly opposite to the previously induced curva- 
ture of the whole growing part for a length of from 7 to 
8 mm. This occurred chiefly when the first curvature 
was small, and when an object had been affixed more 
than, once to the apex of the same radicle. The attach- 
ment of a bit of card by shellac to one side of the tender 
apex may sometimes mechanically^ prevent its growth ; 
or the application of thick gum-water more than once 
to the same side may injure it ; and then checked growth 
on this side with continued growth on the opposite and 
unaffected side would account for the reversed curvature 
of the apex. 

Various trials were made for ascertaining, as far as 
we could, the nature and degree of irritation to which 
the apex must be subjected, in order that the terminal 
growing part should bend away, as if to avoid the cause 
of irritation. We have seen in the numbered experi- 
ments, that a little square of rather thick letter-paper 
gummed to the apex induced, though slowly, consider- 
able deflection. Judging from several cases in which 
various objects had been affixed with gum, and had soon 
become separated from the apex by a layer of fluid, as 
well as from some trials in which drops of thick gum- 
water alone had been applied, this fluid never causes 
bending. We have also seen in the numbered experi- 
ments that narrow splinters of quill and of very thin 
glass, affixed with shellac, caused only a slight degree of 
deflection, and this may perhaps have been due to the 
shellac itself. Little squares of goldbeaters' skin, which 
is excessively thin, were damped, and thus made to ad- 
here to one side of the tipsof two radicles ; one of these, 
after 24 h., produced no effect ; nor did the other in 8 h., 
within which time squares of card usually act ; but after 
24 h. there was slight deflection. 



14:8 SENSITIVENESS OF THE APEX Chap. III. 

An oval bead, or rather cake, of dried shellac, 1.01 
mm. in length and 0-63 in breadth, caused a radicle to 
become deflected at nearly right angles in the course of 
only 6 h. ; but after 23 h. it had nearly straightened 
itself. A very small quantity of dissolved shellac was 
spread over a bit of card, and the tips of 9 radicles were 
touched laterally with it; only two of them became 
slightly deflected to the side opposite to that bearing the 
speck of dried shellac, and they afterwards straightened 
themselves. These specks were removed, and both to- 
gether weighed less than yfo-th of a grain ; so that a weight 
of rather less than 200^^^^ of a grain (0*32 mgs.) sufficed to 
excite movement in two out of the nine radicles. Here 
then we have apparently reached nearly the minimum 
weight which will act. 

A moderately thick bristle (which on measurement 
was found rather flattened, being 0-33 mm. in one diam- 
eter, and 0-20 mm. in the other) was cut into lengths of 
about gig- of an inch. These after being touched with 
thick gum-water, were placed on the tip of eleven radicles. 
Three of them were affected ; one being deflected in 8 
h. 15 m. to an angle of about 90° from the perpendicular: 
a second to the same amount when looked at after 9 h. ; 
but after 24 h. from the time of first attachment the 
deflection had decreased to only 19° ; the third was only 
slightly deflected after 9 h., and the bit of bristle was 
then found not touching the apex ; it was replaced, and 
after 15 additional hours the deflection amounted to 26° 
from the perpendicular. The remaining eight radicles 
were not at all acted on by the bits of bristle, so that we 
here appear to have nearly reached the minimum of size 
of an object which will act on the radicle of the bean. 
But it is remarkable that when the bits of bristle did act, 
that they should have acted so quickly and efficiently. 

As tlic apex of a radicle in penetrating the ground 



Chap. III. OF THE RADICLE OF THE BEAN". 149 

must be pressed on all sides, we wished to learn whether 
it could distinguish between harder or more resisting 
and softer substances. A square of the sanded paper, 
almost as stiff as card, and a square of extremely thin 
paper (too thin for writing on), of exactly the same size 
(about gig-th of an inch), were fixed with shellac on oppo- 
site sides of the apices of 12 suspended radicles. The 
sanded card was between O'lo and 0-20 mm. (or between 
0'0059 and 0"0079 of an inch), and the thin paper only 
0-015 mm. (or 0-00176 of an inch in thickness. In 8 out 
of the 12 cases tliere could be no doubt that the radicle 
was deflected from the side to which the card-like paper 
was attached, and towards the opposite side, bearing the 
very thin paper. This occurred in some instances in 9 
h., but in others not until 21 h. had elapsed. Moreover, 
some of the four failures can hardly be considered as 
really failures : thus, in one of them, in which the radicle 
remained quite straight, the square of thin paper was 
found, when both v/ere removed from the apex, to have 
been so thickly coated with shellac that it was almost as 
stiff as the card : in the second case, the radicle was bent 
upwards into a semicircle, but the deflection was not 
directly from the side bearing the card, and this was 
explained by the two squares having^become cemented 
laterally together, forming a sort of stiff gable, from 
which the radicle was deflected : in the third case, the 
square of card had been fixed by mistake in front, and 
though there was deflection from it, this might have been 
due to Sachs' curvature : in the fourth case alone no 
reason could be assigned why the radicle had not been at 
all deflected. These experiments suffice to prove that 
the apex of the radicle possesses the extraordinary power 
of discriminating between thin card and very thin paper, 
and is deflected from the side pressed by the more resist- 
ing or harder substance. 



150 SENSITIVENESS OF THE APEX Chap. III. 

Some trials were next made by irritating the tips 
without any object being left in contact with them. 
Ninfe radicles, suspended over water, had their tips rubbed, 
each six times with a needle, with sufficient force to 
shake the whole bean ; the temperature was favourable, 
viz. about 63° F. In 7 out of these cases no effect what- 
ever was produced ; in the eighth case the radicle became 
slightly deflected from, and in the ninth case slightly 
deflected towards, the rubbed side ; but these two latter 
opposed curvatures were probably accidental, as radicles 
do not always grow perfectly straight downwards. The 
tips of two other radicles were rubbed in the same manner 
for 15 seconds with a little round twig, two others for 30 
seconds, and two others for 1 minute, but without any 
effect being produced. We may therefore conclude from 
these 15 trials that the radicles are not sensitive to tem- 
porary contact, but are acted on only by prolonged, 
though very slight, pressure. 

We then tried the effects of cutting off a very thin 
slice parallel to one of the sloping sides of the apex, as 
we thought that the wound would cause prolonged irri- 
tation, which might induce bending towards the opposite 
side, as in the case of an attached object. Two prelimi- 
nary trials were made : firstly, slices were cut from the 
radicles of 6 beans suspended in damp air, with a pair of 
scissors, which, though sharp, probably caused consider- 
able crushing, and no curvature followed. Secondly, 
thin slices were cut with a razor obliquely off the tips of 
three radicles similarly suspended ; and after 44 h. two 
were found plainly bent from the sliced surface ; and the 
third, the whole apex of which had been cut off obliquely 
by accident, was curled upwards over the bean, but it was 
not clearly ascertained whether the curvature had been 
at first directed from the cut surface. These results led 
us to pursue the experiment, and 18 radicles, which had 



Chap. III. OF THE RADICLE OP THE BEAN. 151 

grown vertically downwards in damp air, had one side of 
their conical tips sliced off with a razor. The tips were 
allowed just to enter the water in the jars, and they were 
exposed to a temperature 14:°-16° C. (57°-61° F.). The 
observations were made at different times. Three were 
examined 12 h. after being sliced, and were all slightly 
curved from the cut surface ; and the curvature increased 
considerably after an additional 12 h. Eight were ex- 
amined after 19 h. ; four after 22 h. 30 m. ; and three after 
25 h. The final result was that out of the 18 radicles 
thus tried, 13 were plainly bent from the cut surface 
after the above intervals of time ; and one other became 
so after an additional interval of 13 h. 30 m. So that 
only 4 out of the 18 radicles were not acted on. To these 
18 cases the 3 previously mentioned ones should be added. 
It may, therefore, be concluded that a thin slice removed 
by a razor from one side of the conical apex of the radicle 
causes irritation, like that from an attached object, and 
induces curvature from the injured surface. 

Lastly, dry caustic (nitrate of silver) was employed to 
irritate one side of the apex. If one side of the apex or 
of the whole terminal growing part of a radicle is, by 
any means killed or badly injured, the other side con- 
tinues to grow ; and this causes the part to bend over 
towards the injured side.* But in the following experi- 
ments we endeavoured, generally with success, to irri- 
tate the tips on one side, without badly injuring them. 
This was effected by first drying the tip as far as 
possible with blotting-paper, though it still remained 



* Ciesielski found this to be the ponded over water, with a thick 

case ( ' Untersuchungen iiber die layer of grease, which is very in- 

Abwartskriimmung der Wurzel,' jurioijs or even fatal to growing 

1871, p. 28) after burning with parts ; for after 48 hours five of 

heated platinum one side of a these radicleswere curved towards 

radicle. So did we when we the greased side, two remaining 

painted longitudinally half of the straight. 
whole length of 7 radicles, sus- 
11 



152 SENSITIVENESS OF THE APEX Chap. III. 

somewhat damp, and then touching it once with quite 
dry caustic. Seventeen radicles were thus treated, and 
were suspended in moist air over water at a temperature 
of 58° F. They were examined after an interval of 21 h. 
or 24 h. The tips of two were found blackened equally 
all round, so that they could tell nothing and were 
rejected, 15 being left. Of these, 10 were curved from 
the side which had been touched, where there was a 
minute brown or blackish mark. Five of these radicles, 
three of which were already slightly deflected, were 
allowed to enter the water in the jar, and were re-exam- 
ined after an additional interval of 27 h. (i.e. in 48 h. 
after the application of the caustic), and now four of 
them had become hooked, being bent from the dis- 
coloured side with their points directed to the zenith ; 
the fifth remained unaffected and straight. Thus 11 
radicles out of the 15 were acted on. But the curvature 
of the four just described was so plain, that they alone 
would have sufficed to show that the radicles of the bean 
bend away from that side of the apex which has been 
slightly irritated by caustic. 

The poiver of an Irritant on the apex of the Radicle 
of the Bean^ compared with that of Geotropism. — We 
know that when a little square of card or other object is 
fixed to one side of the tip of a vertically dependent 
radicle, the growing part bends from it often into a 
semicircle, in opposition to geotropism, which force is 
conquered by the effect of the irritation from the at- 
tached object. Radicles were therefore extended hori- 
zontally in damp air, kept at the proper low temperature 
for full sensitiveness, and squares of card were affixed 
with shellac on the loiver sides of their tips, so that if 
the squares acted, the terminal growing part would curve 
upwards. Firstly, eight beans were so placed that their 



Chap. IIT. OF THE RADICLE OF THE BEAN. 153 

short, young, horizontally extended radicles would be 
simultaneously acted on both by geotropism and by 
Sachs' curvature, if the latter came into play ; and they 
all eight became bowed downwards to the centre of the 
earth in 20 h., excepting one which was only slightly 
acted on. Two of them were a little bowed downwards 
in only 5 h. ! Therefore the cards, affixed to the lower 
sides of their tips, seemed to produce no effect ; and 
geotropism easily conquered the effects of the irritation 
thus caused. Secondly, 5 oldish radicles, 1-^ inch in 
length, and therefore less sensitive that the above-men- 
tioned young ones, were similarly placed and similarly 
treated. From what has been seen on many other oc- 
casions, it may be safely inferred that if they had been 
suspended vertically they would have bent away from the 
cards ; and if they had been extended horizontally, with- 
out cards attached to them, they would have quickly 
bent vertically downwards through geotropism ; but the 
result was that two of these radicles were still horizontal 
after 23 h. ; two were curved only slightly, and the fifth 
as much as 40° beneath the horizon. Thirdly, 5 beans 
were fastened with their flat surfaces parallel to the cork- 
lid, so that Sachs' curvature would not tend to make the 
horizontally extended radicles turn either upwards or 
downwards, and little squares of card were affixed as 
before, to the lower sides of their tips. The result was 
that all five radicles were bent down, or towards the 
centre of the earth, after only 8 h. 20 m. At the same 
time and within the same jars, 3 radicles of the same 
age, with squares affixed to one side, were suspended 
vertically ; and after 8 h. 20 m. they were considerably 
deflected from the cards, and therefore curved upwards 
in opposition to geotropism. In these latter cases the 
irritation from the squares had overpowered geotropism ; 
whilst in the former cases, in which the radicles were 



154 SENSITIVENESS OF THE RADICLE. Chap. III. 

extended horizontally, geotropism had overpowered the 
irritation. Thus within the same jars, some of the radi- 
cles were curving upwards and others downwards at the 
same time — these opposite movements depending on 
whether the radicles, when the squares were first attached 
to them, projected vertically down, or were extended 
horizontally. This difference in their behaviour seems 
at first inexplicable, but can, we believe, be simply ex- 
plained by the difference between the initial power of 
the two forces under the above circumstances, combined 
with the well-known principle of the after-effects of a 
stimulus. When a young and sensitive radicle is ex- 
tended horizontally, with a square attached to the lower 
side of the tip, geotropism acts on it at right angles, and, 
as we have seen, is then evidently more efficient than 
the irritation from the square ; and the power of geo- 
tropism will be strengthened at each successive period 
by its previous action — that is, by its after-effects. On 
the other hand, when a square is affixed to a vertically 
dependent radicle, and the apex begins to curve upwards, 
this movement will be opposed by geotropism acting 
only at a very oblique angle, and the irritation from the 
card will be strengthened by its previous action. We 
may therefore conclude that the initial power of an irri- 
tant on the apex of the radicle of the bean, is less than 
that of geotropism when acting at right angles, but 
greater than that of geotropism when acting obliquely 
on it. 

Sensitiveness of the tips of the Secondary Radicles of 
the Bean to contact. — All the previous observations relate 
to the main or primary radicle. Some beans suspended 
to cork-lids, with their radicles dipping into water, had 
developed secondary or lateral radicles, which were after- 
wards kept in very damp air, at the proper low tempera- 
ture for full sensitiveness. They projected, as usual. 



Chap. III. SENSITIVENESS OF THE RADICLE. 155 

almost horizontally, with only a slight downward curva- 
ture, and retained this position during several days. 
Sachs has shown * that these secondary roots are acted 
on in a peculiar manner by geotropism, so that if dis- 
placed they reassume their former sub-horizontal posi- 
tion, and do not bend vertically downwards like the 
primary radicle. Minute squares of the stiff sanded paper 
were affixed by means of shellac (but in some instances 
with thick gum- water) to the tips of 39 secondary radi- 
cles of different ages, generally the uppermost ones. 
Most of the squares were fixed to the lower sides of the 
apex, so that if they acted the radicle would bend up- 
wards ; but some were fixed laterally, and a few on the 
upper side. Owing to the extreme tenuity of these radi- 
cles, it was very difficult to attach the square to the 
actual apex. Whether owing to this or some other cir- 
cumstance, only nine of the squares induced any curva- 
ture. The curvature amounted in some cases to about 
45° above the horizon, in others to 90°, and then the tip 
pointed to the zenith. In one instance a distinct up- 
ward curvature was observed in 8 h. 15 m., but usually 
not until 24 h. had elapsed. Although only 9 out of 39 
radicles were affected, yet the curvature was so distinct 
in several of them, that there could be no doubt that 
the tip is sensitive to slight contact, and that the grow- 
ing part bends away from the touching object. It is 
possible that some secondary radicles are more sensitive 
than others ; for Sachs has proved f the interesting fact 
that each individual secondary radicle possesses its own 
peculiar constitution. 

Sensitiveness to contact of the Primary Radicle, a 
little above the apex, in the Bean ( Vicia faha) and Pea 
(Pisum sativum). — The sensitiveness of the apex of the 



* 'Arbeiten Bot. Inst., Wiirz- f 'Arbeiten Bot. Instit., Wiirz- 
burg,' Heft iv. 1874, p. 605-617. burg,' Heft iv. 1874, p. 630. 



156 SENSITIVENESS OF THE Chap. III. 

radicle in the previously described cases, and the conse- 
quent curvature of the upper part from the touching 
object or other source of irritation, is the more remark- 
able, because Sachs * has shown that pressure at the dis- 
tance of a few millimeters above the apex causes the 
radicle to bend, like a tendril, towards the touching object. 
By fixing pins so that they pressed against the radicles 
of beans suspended vertically in damp air, we saw this 
kind of curvature; but rubbing the part with a twig 
or needle for a few minutes produced no effect. Haber- 
landt remarks, f that these radicles in breaking through 
the seed-coats often rub and press against the ruptured 
edges, and consequently bend round them. As little 
squares of the card-like paper affixed with shellac to 
the tips were highly efficient in causing the radicles 
to bend away from them, similar pieces (of about -^^ih 
inch square, or rather less) were attached in the same 
manner to one side of the radicle at a distance of 3 or 4 
mm. above the apex. In our first trial on 15 radicles no 
effect was produced. In a second trial on the same 
number, three became abruptly curved (but only one 
strongly) towards the card within 24 h. From these 
cases we may infer that the pressure from a bit of card 
affixed with shellac to one side above the apex, is hardly 
a sufficient irritant ; but that it occasionally causes the 
radicle to bend like a tendril towards this side. 

We next tried the effect of rubbing several radicles 
at a distance of 4 mm. from the apex for a few seconds 
with lunar caustic (nitrate of silver) ; and although the 
radicles have been wiped dry and the stick of caustic 
was dry, yet the part rubbed was much injured and a 
slight permanent depression was left. In such cases 
the opposite side continues to grow, and the radicle 



'■■■ ' Arbeitcn Bot. lustit., Wiirz- f ' T^i(> Schutzeinrichtungen der 
burg,' Heft iii. 1873, p. 437. Kciinpflanzc,' 1877, p. 25. 



Chap. III. UPPER PART OF THE RADICLE. I57 

necessarily becomes bent towards the injured side. But 
when a point 4 mm. from the apex was momentarily 
touched with dry caustic, it was only faintly discoloured, 
and no permanent injury was caused. This was shown 
by several radicles thus treated straightening themselves 
alter one or two days; yet at first they became curved 
towards the touched side, as if they had been there sub- 
jected to slight continued pressure. These cases deserve 
notice, because when one side of the apex was just 
touched with caustic, the radicle, as we have seen, curved 
itself in an opposite direction, that is, away from the 
touched side. 

The radicle of the common pea at a point a little 
above the apex is rather more sensitive to continued 
pressure than that of the bean, and bends towards the 
pressed side.* We experimented on a variety {Yorh- 
shire Hero) which has a much wrinkled tough skin, 
too large for the included cotyledons; so that out of 30 
peas which had been soaked for 24 h. and allowed to 
germinate on damp sand, the radicles of three were un- 
able to escape, and were crumpled up in a strange manner 
within the skin ; four other radicles were abruptly bent 
round the edges of the ruptured skin against which they 
had pressed. Such abnormalities would probably never, 
or very rarely, occur with forms developed in a state of 
nature and subjected to natural selection. One of the 
four radicles just mentioned in doubling backwards came 
into contact with the pin by which the pea was fixed to 
the cork-lid ; and now it bent at right angles round the 
pin, in a direction quite different from that of the first 
curvature due to contact with the ruptured skin ; and it 
thus afforded a good illustration of the tendril-like sen- 
sitiveness of the radicle a little above the apex. 



Sachs, ' Arbeiten Bot. lustitut., Wiirzburg,' Heft iii. p. 438. 



■158 SENSITIVENESS OF THE APEX Chap. III. 

Little squares of the card-like paper were next affixed 
to radicles of the pea at 4 mm. above the apex, in the 
same manner as with the bean. Twenty-eight radicles 
suspended vertically over water were thus treated on dif- 
ferent occasions, and 13 of them became curved towards 
the cards. The greatest degree of curvature amounted 
to 62° from the perpendicular ; but so large an angle 
was only once formed. On one occasion a slight curva- 
ture was perceptible after 5 h. 45 m., and it was generally 
well-marked after 14 h. There can therefore be no doubt 
that with the pea, irritation from a bit of card attached 
to one side of the radicle above the apex suffices to induce 
curvature. 

Squares of card were attached to one side of the tips 
of 11 radicles within the same jars in which the above 
trials were made, and five of them became plainly, and 
one slightly, curved away from this side. Other analo- 
gous cases will be immediately described. The fact is 
here mentioned because it was a striking spectacle, show- 
ing the difference in the sensitiveness of the radicle in 
different parts, to behold in the same jar one set of radi- 
cles curved away from the squares on their tips, and 
another set curved towards the squares attached a little 
higher up. Moreover, the kind of curvature in the two 
cases is different. The squares attached above the apex 
cause the radicle to bend abruptly, the part above and 
beneath remaining nearly straight; so that here there 
is little or no transmitted effect. On the other hand, 
the squares attached to the apex affect the radicle for a 
length of from about 4 to even 8 mm., inducing in most 
cases a symmetrical curvature ; so that here some in- 
fluence is transmitted from the apex for this distance 
along the radicle. 

Pisum sativum (var. Yorhshire Hero): Sensitiveness 
of the apex of the Eadicle.— Little squares of the same 



Chap. III. OF THE RADICLE OF THE PEA. 



159 



card-like paper were affixed (April 24th) with shellac to 
one side of the apex of 10 vertically suspended radicles : 
the temperature of the water in the bottom of the jars 
was 60°-61° F. Most of these radicles were acted on in 
8 h. 30 m. ; and eight of them became in the course of 
24 h. conspicuously, and the remaining two slightly, de- 
flested from the perpendicular and from the side bearing 
the attached squares. Thus all were acted on ; but it 
will suffice to describe two conspicuous cases. In one 
the terminal portion of the radicle was bent at right 
angles (A, Fig. 66) after 24 h. ; and in the other (B) it 



Fig. 66. 





A B 

Pisum sativum : deflection produced within 24 hours in the growth of 
vertically dependent radicles, by little squares of card affixed with 
shellac to one side of apex : A, bent at right angles ; B, hooked. 

had by this time become hooked, with the apex pointing 
to the zenith. The two bits of card here used were -07 
inch in length and -04 inch in breadth. Two other radi- 
cles, which after 8 h. 30 m. were moderately deflected, 
became straight again after 24 h. Another trial was 
made in the same manner with 15 radicles; but from 
circumstances, not worth explaining, they were only once 
and briefly examined after the short interval of 5 h. 



160 SENSITIVENESS OF THE APEX Chap. III. 

30 m. ; and we merely record in our notes " almost all 
bent slightly from the perpendicular, and away from the 
squares; the deflection amounting in one or two in- 
stances to nearly a rectangle." These two sets of cases, 
especially the first one, prove that the apex of the radi- 
cle is sensitive to slight contact and that the upper part 
bends from the touching object. Nevertheless, on June 
1st and 4th, 8 other radicles were tried in the same man- 
ner at a temperature of 58°-60° F., and after 24 h. only 
1 was decidedly bent from the card, 4 slightly, 2 doubt- 
fully, and 1 not in the least. The amount of curvature 
was unaccountably small ; but all the radicles which were 
at all bent, were bent away from the cards. 

AYe now tried the effects of widely different tempera- 
tures on the sensitiveness of these radicles with squares 
of card attached to their tips. Firstly, 13 peas, most of 
them having very short and young radicles, were placed 
in an ice-box, in which the temperature rose during 
three days from 44° to 47° F. They grew slowly, but 10 
out of the 13 became in the course of the three days very 
slightly curved from the squares ; the other 3 were not 
affected ; so that this temperature was too low for any 
high degree of sensitiveness or for much movement. 
Jars with 13 other radicles were next placed on a chim- 
ney-piece, where they were subjected to a temperature of 
between 68° and 72° F., and after 24 h., 4 were conspicu- 
ously curved from the cards, 2 slightly, and 7 not at all ; 
so that this temperature was rather too high. Lastly, 12 
radicles were subjected to a temperature varying between 
72° and 85° F., and none of them were in the least af- 
fected by the squares. The above several trials, especially 
the first recorded one, indicate that the most favourable 
temperature for the sensitiveness of the radicle of the pea 
is about 00° F. 

The tips of vertically dependent radicles were 



Chap. III. OF THE RADICLE OF THE PEA. 161 

touched once with dry caustic, in the manner described 
under Vicia faba. After 24 h. four of them were bent 
from the side bearing a minute black mark; and the 
curvature increased in one case after 38 h., and in an- 
other case after 48 h., until the terminal part projected 
almost horizontally. The two remaining radicles were 
not affected. 

With radicles of the bean, when extended liorizontally 
in damp air, geotropism always conquered the effects of 
the irritation caused by squares of card attached to the 
lower sides of their tips. A similar experiment was tried 
on 13 radicles of the pea ; the squares being attached 
with shellac, and the temperature between 58°-60° F. 
The result was somewhat different; for these radicles 
are either less strongly acted on by geotropism, or, what 
is more probable, are more sensitive to contact. After a 
time geotropism always prevailed, but its action was 
often delayed; and in three instances there was a most 
curious struggle between geotropism and the irritation 
caused by the cards. Four of the 13 radicles were a 
little curved downwards within 6 or 8 h., always reckon- 
ing from the time when the squares were first attached, 
and after 23 h. three of them pointed vertically down- 
wards, and the fourth at an angle of 45° beneath the 
horizon. These four radicles therefore did not seem to 
have been at all affected by the attached squares. Four 
others were not acted on by geotropism within the first 6 
or 8 h., but after 23 h. were much bowed down. Two 
others remained almost horizontal for 23 h., but after- 
wards were acted on. So that in these latter six cases 
the action of geotropism was much delayed. The 
eleventh radicle Avas slightly curved down after 8 h., but 
when looked at again after 23 h. the terminal portion 
was curved upwards ; if it had been longer observed, the 
tip no doubt would have been found again curved down, 



162 



SENSITIVENESS OF THE APEX Chap. III. 



and it would have formed a loop as in the following case. 
The twelfth radicle after 6 h. was slightly curved down- 
wards ; bnt when looked at again after 21 h., this curva- 
ture had disappeared and the apex pointed upwards; 
after 30 h. the radicle formed a hook, as shown at A 
(Fig. 67) ; which hook after 45 h. was converted into a 
loop (B). The thirteenth radicle after 6 h. was slightly 
curved downwards, but within 21 h. had curved con- 
siderably up, and then down again at an angle of 45° 

Fig. 67. 





Pisum sativum : a radicle extended horizontally in damp air with a 
little square of card affixed to the lower side of its tip, causing it 
to bend upwards in opposition to geotropism. The deflection of 
the radicle after 21 hours is shown at A, and of the same radicle 
after 45 hours at B, now forming a loop. 

beneath the horizon, afterwards becoming perpendicu- 
lar. In these three last cases geotropism and the irrita- 
tion caused by the attached squares alternately prevailed 
in a highly remarkable manner ; geotropism being ulti- 
mately victorious. 

Similar experiments were not always quite so success- 
ful as in the above cases. Thus 6 radicles, horizontally 
extended with attached squares, were tried on June 8th 
at a proper temperature, and after 7 h. 30 m. none were 
in the least curved upwards and none were distinctly 
gcotropic ; whereas of 6 radicles without any attached 
squares, which served as standards of comparison or con- 
trols, 3 became slightly and 3 almost rectangularly geo- 
tropic within the 7 h. 30 m. ; but after 23 h. the two 



Chap. III. OF THE RADICLE OF THE PEA. 163 

lots were equally geotropic. On July 10th another trial 
was made with 6 horizontally extended radicles, with 
squares attached in the same manner beneath their 
tips ; and after 7 h. 30 m., 4 were slightly geotropic, 1 
remained horizontal, and 1 was curved upwards in op- 
position to gravity or geotropism. This latter radicle 
after 48 h. formed a loop, like that at B (Fig. 67). 

An analogous trial was now made, but instead of 
attaching squares of card to the lower sides of the tips, 
these were touched with dry caustic. The details of the 
experiment will be given in the chapter on Geotropism, 
and it will suffice here to say that 10 peas, with radicles 
extended horizontally and not cauterised, were laid on 
and under damp friable peat; these, which served as 
standards or controls, as well as 10 others which had 
been touched on the upper side with the caustic, all 
became strongly geotropic in 24 h. Nine radicles, simi- 
larly placed, had their tips touched on the loiver side 
with the caustic ; and after 24 h., 3 were slightly geo- 
tropic, 2 remained horizontal, and 4 were bowed up- 
wards in opposition to gravity and to geotropism. This 
upward curvature was distinctly visible in 8 h. 45 m. 
after the lower sides of the tips had been cauterised. 

Little squares of card were affixed with shellac on 
two occasions to the tips of 22 young and short sec- 
ondary radicles, which had been emitted from the pri- 
mary radicle whilst growing in water, but were now sus- 
pended in damp air. Besides the difficulty of attaching 
the squares to such finely pointed objects as were these 
radicles, the temperature was too high, — varying on the 
first occasion from 72° to 77° F., and on the second be- 
ing almost steadily 78° F.; and this probably lessened 
the sensitiveness of the tips. The result was that after 
an interval of 8 h. 30 m., 6 of the 22 radicles were 
bowed upwards (one of them greatly) in opposition to 



1G4 SENSITIVENESS OF THE APEX Chap. III. 

gravity, and 2 laterally; the remaining 14 were not 
affected. Considering the unfavourable circumstances, 
and bearing in mind the case of the bean, the evidence 
appears sufficient to show that the tips of the secondary 
radicles of the pea are sensitive to slight contact. 

Phaseolus multifloriis : Sensitivefiess of the apex of 
the Radicle. — Fifty-nine radicles were tried with squares 
of various sizes of the same card-like paper, also with 
bits of thin glass and rough cinders, affixed with shellac 
to one side of the apex. Eather large drops of the 
dissolved shellac were also placed on them and allowed 
to set into hard beads. The specimens were subjected 
to various temperatures between 60° and 72° F., more 
commonly at about the latter. But out of this con- 
siderable number of trials only 5 radicles were plainly 
bent, and 8 others slightly or even doubtfully, from the 
attached objects; the remaining 46 not being at all 
affected. It is therefore clear that the tips of the radi- 
cles of this Phaseolus are much less sensitive to contact 
than are those of the bean or pea. We thought that 
they might be sensitive to harder pressure, but after 
several trials we could not devise any method for press- 
ing harder on one side of the apex than on the other, 
without at the same time offering mechanical resistance 
to its growth. We therefore tried other irritants. 

The tips of 13 radicles, dried with blotting-paper, 
were thrice touched or just rubbed on one side with dry 
nitrate of silver. They were rubbed thrice, because we 
supposed from the foregoing trials, that the tips were 
not highly sensitive. After 24 h. the tips were found 
greatly blackened; 6 were blackened equally all round, 
so that no curvature to any one side could be expected ; 
6 were much blackened on one side for a length of about 
^th of an inch, and this length became curved at right 
angles toiuards the blackened surface, the curvature 



Chap. III. OF THE RADICLE OF PHASEOLUS. 165 

afterwards increasing in several instances until little 
hooks were formed. It was manifest that the blackened 
side was so much injured that it could not grow, whilst 
the opposite side continued to grow. One alone out of 
these 13 radicles became curved from the blackened 
side, the curvature extending for some little distance 
above the apex. 

After the experience thus gained, the tips of six 
almost dry radicles were once touched with the dry 
caustic on one side; and after an interval of 10 m. 
were allowed to enter water, which was kept at a 
temperature of 65°-67° F. The result was that after 
an interval of 8 h. a minute blackish speck could just be 
distinguished on one side of the apex of five of these 
radicles, all of which be(?ame curved towards the oppo- 
site side — in two cases at about an angle of 45° — in two 
other cases at nearly a rectangle — and in the fifth case at 
above a rectangle, so that the apex was a little hooked ; 
in this latter case the black mark was rather larger than 
in the others. After 24 h. from the application of the 
caustic, the curvature of three of these radicles (includ- 
ing the hooked one) had diminished ; in the fourth it 
remained the same, and in the fifth it had increased, the 
tip being now hooked. It has been said that after 8 h. 
black specks could be seen on one side of the apex of five 
of the six radicles ; on the sixth the speck, which was ex- 
tremely minute, was on the actual apex and therefore cen- 
tral ; and this radicle alone did not become curved. It was 
therefore again touched on one side with caustic, and 
after 15 h. 30 m. was found curved from the perpen- 
dicular and from the blackened side at an angle of 34°, 
which increased in nine additional hours to 54°. 

It is therefore certain that the apex of the radicle 
of this Phaseolus is extremely sensitive to caustic, 
more so than that of the bean, though the latter is 



166 SENSITIVENESS OF THE APEX Chap. III. 

far more sensitive to pressure. In the experiments 
just given, the curvature from the sliglitly cauterised 
side of the tip, extended along the radicle for a 
length of nearly 10 mm. ; whereas in the first set 
of experiments, when the tips of several were greatly 
blackened and injured on one side, so that their growth 
was arrested, a length of less than 3 mm. became 
curved toiuards the much blackened side, owing to the 
continued growth of the opposite side. This differ- 
ence in the results is interesting, for it shows that too 
strong an irritant does not induce any transmitted 
effect, and does not cause the adjoining, upper and 
growing part of the radicle to bend. We have analo- 
gous cases with Drosera, for a strong solution of car- 
bonate of ammonia when absorbed by the glands, or 
too great heat suddenly applied to them, or crushing 
them, does not cause the basal part of the tentacles 
to bend, whilst a weak solution of the carbonate, or a 
moderate heat, or slight pressure always induces such 
bending. Similar results were observed with Dionaea 
and Pinguicula. 

The effect of cutting off with a razor a thin slice 
from one side of the conical apex of 14 young and short 
radicles was next tried. Six of them after being oper- 
ated on were suspended in damp air; the tips of the 
other eight, similarly suspended, were allowed to enter 
water at a temperature of about 65° F. It was recorded 
in each case which side of the apex had been sliced off, 
and when they were afterwards examined the direction 
of the curvature was noted, before the record was con- 
sulted. Of the six radicles in damp air, three had their 
tips curved after an interval of 10 h. 15 m. directly away 
from the sliced surface, whilst the other three were not 
affected and remained straight; nevertheless, one of 
them after 13 additional hours became slightly curved 



Chap. III. OF THE RADICLE OF TROP^OLUM. 167 

from the sliced surface. Of the eight radicles with their 
tips immersed in water, seven were plainly curved away 
from the sliced surfaces after 10 h. 15 m. ; and with 
respect to the eighth which remained quite straight, 
too thick a slice had been accidentally removed, so that 
it hardly formed a real exception to the general result. 
When the seven radicles were looked at again, after an 
interval of 23 h. from the time of slicing, two had be- 
come distorted ; four were deflected at an angle of about 
70° from the perpendicular and from the cut surface ; 
and one was deflected at nearly 90°, so that it projected 
almost horizontally, but with the extreme tip now begin- 
ning to bend downwards through the action of geotro- 
pism. It is therefore manifest that a thin slice cut off 
one side of the conical apex, causes the upper growing 
part of the radicle of this Phaseolus to bend, through 
the transmitted effects of the irritation, away from the 
sliced surface. 

TropcBolum majus : Sensitiveness of the apex of the 
Radicle to contact. — Little squares of card were attached 
with shellac to one side of the tips of 19 radicles, some 
of which were subjected to 78° F., and others to a much 
lower temperature. Only 3 became plainly curved from 
the squares, 5 slightly, 4 doubtfully, and 7 not at all. 
These seeds were, as we believed, old, so we procured a 
fresh, lot, and now the results were widely different. 
Twenty-three were tried in the same manner ; five of 
the squares produced no effect, but three of these cases 
were no real exceptions, for in two of them the squares 
had slipped and were parallel to the apex, and in the 
third the shellac was in excess and had spread equally 
all round the apex. One radicle was deflected only 
slightly from the perpendicular and from the card ; 
whilst seventeen were plainly deflected. The angles in 

several of these latter cases varied between 40° and 05° 
12 



168 SENSITIVENESS OF THE APEX Chap. III. 

from the perpendicular ; and in two of them it amounted 
after 15 h. or 16 h. to about 90°. In one instance a loop 
was nearly completed in 16 h. There can, therefore, be 
no doubt that the apex is highly sensitive to slight con- 
tact, and that the upper part of the radicle bends away 
from the touching object. 

Gossypiimi herhaceum : Sensitiveness of the apex of 
the Radicle. — Eadicles were experimented on in the 
same manner as before, but they proved ill-fitted for our 
purpose, as they soon became unhealthy when suspended 
in damp air. Of 38 radicles thus suspended, at temper- 
atures varying from 66° to 69° F., with squares of card 
attached to their tips, 9 were plainly and 7 slightly or 
even doubtfully deflected from the squares and from the 
perpendicular ; 22 not being affected. We thought that 
perhaps the above temperature was not high enough, so 
19 radicles with attached squares, likewise suspended in 
damp air, were subjected to a temperature of from 74° 
to 79° F., but not one of them was acted on, and they 
soon became unhealthy. Lastly, 19 radicles were sus- 
pended in water at a temperature from 70° to 75° F., 
with bits of glass or squares of the card attached to their 
tips by means of Canada-balsam or asphalte, which ad- 
hered rather better than shellac beneath the water. The 
radicles did not keep healthy for long. The result was 
that 6 were plainly and 2 doubtfully deflected from the 
attached objects and the perpendicular ; 11 not being 
affected. The evidence consequently is hardly conclu- 
sive, though from the two sets of cases tried under a 
moderate temperature, it is probable that the radicles 
are sensitive to contact ; and would be more so under 
favourable conditions. 

Fifteen radicles which had germinated in friable 
peat were suspended vertically over water. Seven of 
them served as controls, and they remained quite straight 



Chap. III. OF THE RADICLE OF CUCURBITA. 169 

during 24 h. The tips of the other eight radicles were 
just touched with dry caustic on one side. After only 5 
h. 10 m. five of them were slightly curved from the per- 
pendicular and from the side bearing the little blackish 
marks. After 8 h. 40 m., 4 out of these 5 were deflected 
at angles between 15° and 65° from the perpendicular. 
On the other hand, one which had been slightly curved 
after 5 h. 10 m., now became straight. After 24 h. the 
curvature in two cases had considerably increased ; also 
in four other cases, but these latter radicles had now be- 
come so contorted, some being turned upwards, that it 
could no longer be ascertained whether they were still 
curved from the cauterised side. The control specimens 
exhibited no such irregular growth, and the two sets pre- 
sented a striking contrast. Out of the 8 radicles which 
had been touched with caustic, two alone were not 
affected, and the marks left on their tips by the caustic 
were extremely minute. These marks in all cases were 
oval or elongated ; they were measured in three instances, 
and found to be of nearly the same size, viz. f of a mm. 
in length. Bearing this fact in mind, it should be observed 
that the length of the curved part of the radicle, which 
had become deflected from the cauterised side in the 
course of 8 h. 40 m., was found to be in three cases 6, 7, 
and 9 mm. 

Cucurhita ovifera: Sensitiveness of the apex of the 
Radicle. — The tips proved ill-fitted for the attachment 
of cards, as they are extremely fine and flexible. More- 
over, owing to the hypocotyls being soon developed and 
becoming arched, the whole radicle is quickly displaced 
and confusion is thus caused. A large number of trials 
were made, but without any definite result, excepting 
on two occasions, when out of 23 radicles 10 were de- 
flected from the attached squares of card, and 13 were 



170 SENSITIVENESS OP THE APEX Chap. III. 

not acted on. Eather large squares, though difficult to 
affix, seemed more efficient than very small ones. 

We were much more successful with caustic; but in 
our first trial, 15 radicles were too much cauterised, and 
only two became curved from the blackened side; the 
others being either killed on one side, or blackened 
equally all round. In our next trial the dried tips of 11 
radicles were touched momentarily with dry caustic, and 
after a few minutes were immersed in water. The elon- 
gated marks thus caused were never black, only brown, 
and about ^ mm. in length, or even less. In 4 h. 30 m. 
after the cauterisation, 6 of them were plainly curved 
from the side with the brown mark, 4 slightly, and 1 not 
at all. The latter proved unhealthy, and never grew ; 
and the marks on 2 of the 4 slightly curved radicles were 
excessively minute, one being distinguishable only with 
the aid of a lens. Of 10 control specimens tried in 
the same jars at the same time, not one was in the 
least curved. In 8 h. 40 m. after the cauterisation, 5 
of the radicles out of the 10 (the one unhealthy one 
being omitted) were deflected at about 90°, and 3 at 
about 45° from the perpendicular and from the side 
bearing the brown mark. After 24 h. all 10 radicles 
had increased immensely in length; in 5 of them the 
curvature was nearly the same, in 2 it had increased, and 
in 3 it had decreased. The contrast presented by the 10 
controls, after both the 8 h. 40 m. and the 24 h. inter- 
vals was very great ; for they had continued to grow ver- 
tically downwards, excepting two which, from some 
unknown cause, had become somewhat tortuous. 

• In the chapter on Geotropism, we shall see that 10 
radicles of this plant were extended horizontally on and 
beneath damp friable peat, under which conditions they 
grow better and more naturally than in damp air ; and 
their tips were slightly cauterised on the lower side, 



Chap. III. OF THE RADICLE OP ^SCULUS. l71 

brown marks about ^ mm- in length being thus caused. 
Uncauterised specimens similarly placed became much 
bent downwards through geotropism in the course of 5 
or 6 hours. After 8 h. only 3 of the cauteriesd ones 
were bowed downwards, and this in a slight degree ; 4 
remained horizontal; and 3 were curved upwards in 
opposition to geotropism and from the side bearing 
the brown mark. Ten other specimens had their tips 
cauterised at the same time and in the same degree, 
on the upper side ; and this, if it produced any effect, 
would tend to increase the power of geotropism ; and all 
these radicles were strongly bowed downwards after 8 h. 
From the several foregoing facts, there can be no doubt 
that the cauterisation of the tip of the radicle of this 
Oncurbita on one side, if done lightly enough, causes the 
whole growing part to bend to the opposite side. 

Raphanus sativus : Sensitiveness of the apex of the 
Radicle. — We here encountered many difficulties in our 
trials, both with squares of card and with caustic ; for 
when seeds were pinned to a cork-lid, many of the radicles, 
to which nothing had been done, grew irregularly, often 
curving upwards, as if attracted by the damp surface 
above ; and when they were immersed in water they like- 
wise often grew irregularly. We did not therefore dare 
to trust our experiments with attached squares of card; 
nevertheless some of them seemed to indicate that the 
tips were sensitive to contact. Our trials with caustic 
generally failed from the difficulty of not injuring too 
greatly the extremely fine tips. Out of 7 radicles thus 
tried, one became bowed after 22 h. at an angle of 60°, 
a second at 40°, and a third very slightly from the per- 
pendicular and from the cauterised side. 

^senilis hijypocastanum : Sensitiveness of the apex 
of the Radicle. — Bits of glass and squares of card were 
affixed with shellac or gum-water to the tips of 12 radicles 



172 SENSITIVENESS OF THE APEX Chap. III. 

of the horse-chestnut ; and when these objects fell off, 
they were refixed ; but not in a single instance was 
any curvature thus caused. These massive radicles, one 
of which was above 2 inches in length and '3 inch in 
diameter at its base, seemed insensible to so slight a 
stimulus as any small attached object. Nevertheless, 
when the apex encountered an obstacle in its downward 
course, the growing part became so uniformly and sym- 
metrically curved, that its appearance indicated not mere 
mechanical bending, but increased growth along the 
whole convex side, due to the irritation of the apex. 

That this is the correct view may be inferred from 
the effects of the more powerful stimulus of caustic. The 
bending from the cauterised side occurred much slower 
than in the previously described species, and it will per- 
haps be worth while to give our trials in detail. 

The seeds germinated in sawdust, and one side of the tips 
of the radicles were slightly rubbed once with dry nitrate of 
silver ; and after a few minutes were allowed to dip into water. 
They were subjected to a rather varying temperature, generally 
between 52° and 58° F. A few cases have not been thought 
worth recording, in which the whole tip was blackened, or in 
which the seedling soon became unhealthy. 

(1.) The radicle, was slightly deflected from the cauterised 
side in one day (i.e. 24 h.) ; in three days it stood at 60° from 
the perpendicular; in four days at 90°; on the fifth day it was 
curved up about 40° above the horizon; so that it had passed 
through an angle of 130° in the five days, and this was the 
greatest amount of curvature observed. 

(2.) In two days radicle slightly deflected ; after seven days 
deflected 69° from the perpendicular and from the cauterised 
side ; after eight days the angle amounted to nearly 90°. 

(3.) After one day slight deflection, but the cauterised mark 
was so faint that the same side was again touched with caustic. 
In four days from the first touch deflection amounted to 78", 
which in an additional day increased to 90°. 



Chap. III. OF THE RADICLE OF iESCULUS. 173 

(4.) After two days slight deflection, which during the next 
three days certainly increased but never became great; the 
radicle did not grow well and died on the eighth day. 

(5.) After two days very slight deflection; but this on the 
fourth day amounted to 56° from the perpendicular and from 
the cauterised side. 

(6.) After three days doubtfully, but after four days certainly 
deflected from the cauterised side. On the fifth day deflection 
amounted to 45° from the perpendicular, and this on the seventh 
day increased to about 90°. 

(7.) After two days slightly deflected; on the third day the 
deflection amounted to 25° from the perpendicular, and this did 
not afterwards increase. 

(8.) After one day deflection distinct; on the third day it 
amounted to 44°, and on the fourth day to 72° from the perpen- 
dicular and the cauterised side. 

(9.) After two days deflection slight, yet distinct; on the 
third day the tip was again touched on the same side with 
caustic and thus killed. 

(10.) After one day slight deflection, which after six days 
increased to 50° from the perpendicular and the cauterised side. 

(11.) After one day decided deflection, which after six days 
increased to 62° from the perpendicular and from the cauterised 
side. 

(12.) After one day slight deflection, which on the second 
day amounted to 35°, on the fourth day to 50°, and the sixth 
day to 63° from the perpendicular and the cauterised side. 

(13.) Whole tip blackened, but more on one side than the 
other; on the fourth day slightly, and on the sixth day greatly 
deflected from the more blackened side ; the deflection on the 
ninth day amounted to 90° from the perpendicular. 

(14.) Whole tip blackened in the same manner as in the last 
case; on the second day decided deflection from the more 
blackened side, which increased on the seventh day to nearly 
90° ; on the following day the radicle appeared unhealthy. 

(15.) Here we had the anomalous case of a radicle bending 
slightly towards the cauterised side on the first day, and con- 
tinuing to do so for the next three days, when the deflection 
amounted to about 90° from the perpendicular. The cause 



174 SENSITIVENESS OF THE APEX Chap. III. 

appeared to lie in the tendril-like sensitiveness of the upper 
part of the radicle, against which the point of a large triangular 
flap of the seed-coats pressed with considerable force; and 
this irritation apparently conquered that from the cauterised 
apex. 

These several cases show beyond doubt that the irri- 
tation of one side of the apex, excites the upper part of 
the radicle to bend slowly towards the opposite side. 
This fact was well exhibited in one lot of fi.ve seeds 
pinned to the cork-lid of a jar ; for when after 6 days 
the lid was turned upside down and viewed from directly 
above, the little black marks made by the caustic were 
now all distinctly visible on the upper sides of the tips of 
the laterally bowed radicles. 

A thin slice was shaved off with a razor from one side 
of the tips of 22 radicles, in the manner described under 
the common bean ; but this kind of irritation did not 
prove very effective. Only 7 out of the 22 radicles be- 
came moderately deflected in from 3 to 5 days from the 
sliced surface, and several of the others grew irregularly. 
The evidence, therefore, is far from conclusive. 

Quercus rohur : Sensitiveness of the apex of the 
Radicle. — The tips of the radicles of the common oak are 
fully as sensitive to slight contact as are those of any 
plant examined by us. They remained healthy in damp 
air for 10 days, but grew slowly. Squares of the card- 
like paper were fixed with shellac to the tips of 15 radicles, 
and ten of these became conspicuously bowed from the 
perpendicular and from the squares ; two slightly, and 
three not at all. But two of the latter were not real ex- 
ceptions, as they were at first very short, and hardly 
grew afterwards. Some of the more remarkable cases 
are worth describing. The radicles were examined on 
each successive morning, at nearly the same hour, that 
is, after intervals of 24 h. 



Chap. III. OB^ THE RADICLE OF QUERCUS. 



1Y5 



Fig. 68. 



No. 1. This radicle suffered from a series of accidents, and 
acted in an anomalous manner, for the apex appeared at first 
insensible and afterwards sensitive to contact. The first square 
was attached on Oct. 19th ; on the 21st the radicle was not at 
all curved, and the square was accidentally knocked off; it was 
refixed on the 22nd, and the radicle became slightly curved from 
the square, but the curvature disappeared on the 23rd, when the 
square was removed and refixed. No curvature ensued, and the 
square was again accidentally knocked off, and refixed. On the 
morning of the 27th it was washed off by 
having reached the water in the bottom 
of the jar. The square was refixed, and 
on the 29th, that is, ten days after the 
first square had been attached, and two 
days after the attachment of the last 
square, the radicle had grown to the great 
length of 3'2 inches, and now the terminal 
growing part had become bent away from 
the square into a hook (see Fig. 68). 

No. 2. Square attaclied on the 19th; 
on the 20th radicle slightly deflected from 
it and from the perpendicular; on the 
21st deflected at nearly right angles; it 
remained during the next two days in 
this position, but on the 25th the up- 
ward curvature was lessened through the 
action of geotropism, and still more so 
on the 2Gth. 

No. 3. Square attached on the 19th; 
on the 21st a trace of curvature from 
amounted on the 22nd to about 40°, and 
from the perpendicular. 

No. 4. Square attached on the 21st; on the 22nd trace of 
curvature from the square ; on the 23rd completely hooked with 
the point turned up to the zenith. Three days afterwards (i.e. 
26th) the curvature had wholly disappeared and the apex pointed 
perpendicularly downwards. 

No. 5. Square attached on the 21st; on the 22nd decided 
though slight curvature from the square ; on the 23rd the tip 




Quercus rohir : radicle 
with square of card 
attached to one side 
of apex, causing it 
to become hooked. 
Drawing one - half 
natural scale. 

the square, which 
on the 23rd to 53" 



176 SENSITIVENESS OF THE APEX Chap. III. 

liad curved up above the horizon, and on the 24th was hooked 
with the apex pointing almost to the zenith, as in Fig. 68. 

No. 6. Square attached on the 21st; on the 22nd slightly 
curved from the square ; 23rd more curved ; 25th considerably 
curved ; 27th all curvature lost, and the radicle was now directed 
perpendicularly downwards. 

No. 7. Square attached on the 21st; on the 22nd a trace of 
curvature from the square, which increased next day, and on 
the 24th amounted to a right angle. 

It is, therefore, manifest that the apex of the radicle 
of the oak is highly sensitive to contact, and retains its 
sensitiveness during several days. The movement thus 
induced was, however, slower than in any of the previous 
cases, with the exception of that of ^sculus. As with 
tlie bean, the terminal growing part, after bending, some- 
times straightened itself through the action of geotro- 
pism, although the object still remained attached to the 
tip. 

The same remarkable experiment was next tried, as 
in the case of the bean ; namely, little squares of exactly 
the same size of the card-like sanded paper and of very 
thin paper (the thickness of which have been given under 
Vicia faha) were attached with shellac on opposite sides 
(as accurately as could be done) of the tips of 13 radicles, 
suspended in damp air, at a temperature of 65°-66° F. 
The result was striking, for 9 out of these 13 radicles be- 
came plainly, and one very slightly, curved from the thick 
paper towards the side bearing the thin paper. In two 
of these cases the apex became completely hooked after 
two days ; in four cases the deflection from the perpen- 
dicular and from the side bearing the thick paper, 
amounted in from two to four days to angles of 90°, 72°, 
G0°, and 49°, but in two other cases to only 18° and 15°. 
It should, however, be stated that in the case in which 
the deflection was 49°, the two squares had accidentally 



Chap. III. OF THE RADICLE OF ZEA. 17Y 

come into contact on one side of the apex, and thus 
formed a lateral gable ; and the deflection was directed. 
in part from this gable and in part from tlie thick paper. 
In three cases alone the radicles were not affected by the 
difference in thickness of the squares of paper attached 
to their tips, and consequently did not bend away from 
the side bearing the stiffer paper. 

Zea mays : Sensitiveness of the apex of the Radicle to 
contact. — A large number of trials were made on this 
plant, as it was the only monocotyledon on w^hich we ex- 
perimented. An abstract of the results will suffice. In 
the first place, 22 germinating seeds were pinned to cork- 
lids without any object being attached to their radicles, 
some being exposed to a temperature of 65°-66° F., and 
others to between 74° and 79° ; and none of them be- 
came curved, though some were a little inclined to one 
side. A few were selected, which from having germi- 
nated on sand were crooked, but when suspended in 
damp air the terminal part grew straight downwards. 
This fact having been ascertained, little squares of the 
card-like paper were affixed with shellac, on several oc- 
casions, to the tips of 68 radicles. Of these the terminal 
growing part of 39 became within 24 h. conspicuously 
curved away from the attached squares and from the per- 
pendicular ; 13 out of the 39 forming hooks with their 
points directed towards the zenith, and 8 forming loops. 
Moreover, 7 other radicles out of the 68, were slightly and 
two doubtfully deflected from the cards. There remain 
20 which were not affected ; but 10 of these ought not 
to be counted ; for one was diseased, two had their tips 
quite surrounded by shellac, and the squares on 7 had 
slipped so as to stand parallel to the apex, instead of 
obliquely on it. There were therefore only 10 out of the 
68 which certainly were not acted on. Some of the radi- 
cles which were experimented on were young and short, 



178 SENSITIVENESS OF THE APEX Chap. III. 

most of them of moderate length, and two or three ex- 
ceeded three inches in length. The curvature in the 
above cases occurred within 24 h., but it was often con- 
spicuous within a much shorter period. For instance, 
the terminal growing part of one radicle was bent up- 
wards into a rectangle in 8 h. 15 m., and of another in 
9 h. On one occasion a hook was formed in 9 h. Six 
of the radicles in a jar containing nine seeds, which stood 
on a sand-bath, raised to a temperature varying from 76° 
to 82° F., became hooked, and a seventh formed a com- 
plete loop, when first looked at after 15 hours. 

The accompanying figures of four germinating seeds 
(Fig. 69) show, firstly, a radicle (A) the apex of which 
has become so much bent away from the attached square 
as to form a hook. Secondly (B), a hook converted 
through the continued irritation of the card, aided per- 
haps by geotropism, into an almost complete circle or 
loop. The tip in the act of forming a loop generally 
rubs against the upper part of the radicle, and pushes off 
the attached square ; the loop then contracts or closes, 
but never disappears ; and the apex afterwards grows ver- 
tically downwards, being no longer irritated by any at- 
tached object. This frequently occurred, and is repre- 
sented at 0. The jar above mentioned with the six 
hooked radicles and another jar were kept for two addi- 
tional days, for the sake of observing how the hooks 
would be modified. Most of them became converted into 
simple loops, like that figured at ; but in one case the 
apex did not rub against the upper part of the radicle 
and thus remove the card ; and it consequently made, 
owing to the continued irritation from the card, two 
complete loops, that is, a helix of two spires ; which after- 
wards became pressed closely together. Then geotro- 
pism prevailed and caused the apex to grow perpendicu- 
larly dowuAvards. In another case, shown at (D), the 



Chap. III. 



OF THE RADICLE OF ZEA. 



179 



apex in making a second turn or spire passed through 
the first loop, which was at first widely open, and in 
doing so knocked olf the card ; it then grew perpendicu- 
larly downwards, and thus tied itself into a knot, which 
soon became tight ! 

Fig. 69. 







C D 

Zea mays: radicles excited to bend away from the little squares of 
card attached to one side of their tips. 



Secondary Radicles of Zea. — A short time after the 
first radicle has appeared, others protrude from the seed, 
but not laterally from the primary one. Ten of these 
secondary radicles, which were directed obliquely down- 
wards, were experimented on with very small squares of 
card attached with shellac to the lower sides of their 



180 SENSITIVENESS OP THE APEX Chap. III. 

tips. If therefore the squares acted, the radicles would 
bend upwards in opposition to gravity. The jar stood 
(protected from light) on a sand-bath, which varied be- 
tween 76° and 82° F. After only 5 h. one appeared to 
be a little deflected from the square, and after 20 h. 
formed a loop. Four others were considerably curved 
from the squares after 20 h., and three of them became 
hooked, with their tips pointing to the zenith, — one after 
29 h. and the two others after 44 h. By this latter time 
a sixth radicle had become bent at a right angle from 
the side bearing the square. Thus altogether six out of 
the ten secondary radicles were acted on, four not being 
affected. There can, therefore, be no doubt that the tips 
of these secondary radicles are sensitive to slight contact, 
and that when thus excited they cause the upper part to 
bend from the touching object ; but generally, as it ap- 
pears, not in so short a time as in the case of the first- 
formed radicle. 

Sensitive:n^ess of the tip of the Radicle 
TO Moist Air. 

Sachs made the interesting discovery, a few years 
ago, that the radicles of many seedling plants bend to- 
wards an adjoining damp surface.* We shall here en- 
deavour to show that this peculiar form of sensitiveness 
resides in their tips. The movement is directly the 
reverse of that excited by the irritants hitherto consid- 
ered, which cause the growing part of the radicle to bend 
away from the source of irritation. In our experiments 
we followed Sachs' plan, and sieves with seeds germinat- 
ing in damp sawdust were suspended so that the bottom 
was generally inclined at 40° with the horizon. If the 
radicles had been acted on solely by geotropism, they 

* ' Arbciten des Bot. Institut., in Wiirzburg,' vol. i. 1872, p. 209. 



Chap. III. OF THE RADICLE TO MOIST AIR. 181 

would have grown out of the bottom of the sieve perpen- 
dicularly downwards ; but as they were attracted by the 
adjoining damp surface they bent towards it and were 
deflected 50° from the perpendicular. For the sake of 
ascertaining whether the tip or the whole growing part 
of the radicle was sensitive to the moist air, a length of 
from 1 to 2 mm. was coated in a certain number of cases 
with a mixture of olive-oil and lamp-black. This mix- 
ture was made in order to give consistence to the oil, so 
that a thick layer could be applied, which would exclude, 
at least to a large extent, the moist air, aud would be 
easily visible. A greater number of experiments than 
those which were actually tried would have been neces- 
sary, had not it been clearly established that the tip of 
the radicle is the part which is sensitive to various other 
irritants. 

Phaseolus multijiorus. — Twenty-nine radicles, to which noth- 
ing had been done, growing out of a sieve, were observed at 
the same time with those which had their tips greased, and for 
an equal length of time. Of the 29, 24 curved themselves so as 
to come into close contact with the bottom of the sieve. The 
place of chief curvature was generally at a distance of 5 or 6 
mm. from the apex. Eight radicles had their tips greased for 
a length of 2 mm., and two others for a length of 1^ mm. ; they 
were kept at a temperature of 15°-16° C. After intervals of 
from 19 ]i. to 24 h. all were still vertically or almost vertically 
dependent, for some of them had moved towards the adjoining 
damp surface by about 10°. They had therefore not been acted 
on, or only slightly acted on, by the damper air on one side, 
although the whole upper part was freely exposed. After 48 
h. three of these radicles became considerably curved towards 
the sieve ; and the absence of curvature in some of the others 
might perhaps be accounted for by their not having grown very 
well. But it should be observed that during the first 19 h. to 
24 h. all grew w^ell ; two of them having increased 2 and 3 mm. 
in length in 11 h. ; five others increased 5 to 8 mm. in 19 h. ; 



182 SENSITIVENESS OF THE APEX Chap. III. 

and two, which had been at first 4 and 6 mm. in length, in- 
creased in 24 h. to 15 and 20 mm. 

The tips of 10 radicles, which likewise grew well, were 
coated with the grease for a length of only 1 mm., and now 
the result was somewhat different ; for of these 4 curved them- 
selves to the sieve in from 21 h. to 24 h., whilst 6 did not do 
so. Five of the latter were observed for an additional day, and 
now all excepting one became curved to the sieve. 

The tips of 5 radicles were cauterised with nitrate of silver, 
and about 1 mm. in length was thus destroyed. They were 
observed for periods varying between 11 h. and 24 h., and were 
found to have grown well. One of them had curved until it 
came into contact with the sieve ; another was curving towards 
it; whilst the remaining three were still vertically dependent. 
Of 7 not cauterised radicles observed at the same time, all had 
come into contact with the sieve. 

The tips of 11 radicles were protected by moistened gold- 
beaters' skin, which adheres closely, for a length varying from 
1^ to 2 J mm. After 22 h. to 24 h., 6 of these radicles were 
clearly bent towards or had come into contact witli the sieve; 
2 were slightly curved in this direction, and 3 not at all. All 
had grown well. Of 14 control specimens observed at the same 
time, all excepting one had closely apjDroached the sieve. It 
appears from these cases that a cap of goldbeaters' skin checks, 
though only to a slight degree, the bending of the radicles to 
an adjoining damp surface. Whether an extremely thin sheet 
of this substance when moistened allows moisture from the air 
to pass through it, we do not know. One case indicated that 
the caps were sometimes more eflBcient than appears from the 
above results ; for a radicle, which after 23 h. had only slightly 
approached the sieve, had its cap (1^ mm. in length) removed, 
and during the next 15| h. it curved itself abruptly towards 
the source of moisture, the chief seat of curvature being at a 
distance of 2 to 3 mm. from the apex. 

Viciafaba. — The tips of 13 radicles were coated with the 
grease for a length of 2 mm.; and it should be remembered 
that witli these radicles the seat of chief curvature is about 
4 or 5 mm. from the apex. Four of them were examined after 
22 h., three after 20 h., and six after 36 h., and none had been 



Chap. III. OF THE RADICLE TO MOIST AIR. 183 

attracted towards the damp lower surface of the sieve. In an- 
other trial 7 radicles were similarly treated, and 5 of them still 
pointed perpendicularly downwards after 11 h., whilst 3 were 
a little curved towards the sieve ; by an accident they were not 
subsequently observed. In both these trials the radicles grew 
well; 7 of them, which were at first from 4 to 11 mm. in length, 
were after 11 h. between 7 and 16 mm. ; 3 which were at first 
from 6 to 8 mm. after 36 h. were 115 to 18 mm. in length; and 
lastly, 4 radicles which were at first 5 to 8 mm. after 46 h. were 
18 to 33 mm. in length. The control or ungreased radicles were 
not invariably attracted towards the bottom of the sieve. But 
on one occasion 13 out of 13, which were observed for periods 
between 33 h. and 36 h., were thus attracted. On two other 
occasions taken together, 38 out of 40 were similarly attracted. 
On another occasion only 7 out of 14 behaved in this manner, 
but after two more days the proportion of the curved increased 
to 17 out of 33. On a last occasion only 11 out of 30 were thus 
attracted. If we add up these numbers, we find that 78 out of 
96 of the control specimens curved themselves towards the bot- 
tom of the sieve. Of the specimens with greased tips, 3 alone 
out of the 30 (but 7 of these were not observed for a suflBciently 
long time) thus curved themselves. We can, therefore, hardly 
doubt that the tip for a length of 3 mm. is the part which is 
sensitive to a moist atmosphere, and causes the upper part to 
bend towards its source. 

The tips of 15 radicles were cauterised with nitrate of silver, 
and they grew as well as those above described with greased 
tips. After an interval of 34 h., 9 of them were not at all 
curved towards the bottom of the sieve ; 3 were curved towards 
it at angles of 30° and 13° from their former vertical position, 
and 4 had come into close contact with it. Thus the destruc- 
tion of the tip for a length of about 1 mm. prevented the curva- 
ture of the greater number of these radicles to the adjoining 
damp surface. Of 24 control specimens, 33 were bent to the 
sieve, and on a second occasion 15 out of 16 were similarly 
curved in a greater or less degree. These control trials are 
included in those given in the foregoing paragraph. 

Avena sativa.— The tips of 13 radicles, which projected 
between 3 and 4 mm. from the bottom of the sieve, many of, 



184 SENSITIVENESS OF THE APEX. Chap. III. 

them not quite perpendicularly downwards, were coated with 
the black grease for a length of from 1 to li mm. The sieves 
were inclined at 30° with the horizon. The greater number of 
these radicles were examined after 22 h., and a few after 25 h , 
and within these intervals they had grown so quickly as to have 
nearly doubled their lengths. With the ungreased radicles the 
chief seat of curvature is at a distance of not less than between 
3'5 and 5-5 mm., aind not more than between 7 and 10 mm. from 
the apex. Out of the 13 radicles with greased tips, 4 had not 
moved at all towards the sieve ; 6 were deflected towards it and 
from the perpendicular by angles varying between 10° and 35°; 
and 3 had come into close contact with it. It appears, there- 
fore, at first sight that greasing the tips of these radicles had 
checked but little their bending to the adjoining damp surface. 
But the inspection of the sieves on two occasions produced a 
widely different impression on the mind ; for it was impossible 
to behold the radicles with the black greased tips projecting 
from the bottom, and all those with ungreased tips, at least 40 
to 50 in number, clinging closely to it, and feel any doubt that 
the greasing had produced a great effect. On close examina- 
tion only a single ungreased radicle could be found which had 
not become curved towards the sieve. It is probable that if the 
tips had been protected by grease for a length of 2 mm. instead 
of from 1 to 1^ mm., they would not have been affected by the 
moist air and none would have become curved. 

Triticum vulgare. — Analogous trials were made on 8 radicles 
of the common wheat ; and greasing their tips produced much 
less effect than in the case of the oats. After 22 h., 5 of them 
had come into contact with the bottom of the sieve; 2 had 
moved towards it 10° and 15°, and one alone remained perpen- 
dicular. Not one of the very numerous ungreased radicles 
failed to come into close contact with the sieve. These trials 
were made on Nov. 28th, when the temperature was only 4° -8 
C. at 10 A.M. We should hardly have thought this case worth 
notice, had it not been for the following circumstance. In the 
beginning of October, when the temperature was considerably 
higher, viz., 12° to 13° C, we found that only a few of the un- 
greased radicles became bent towards the sieve ; and this indi- 
cates that sensitiveness to moisture in the air is increased by a 



Chap. III. OF THE RADICLE TO MOIST AIR. 185 

low temperature, as we have seen with the radicles of Vicia 
faba relatively to objects attached to their tips. But in the 
present instance it is possible that a difference in the dryness of 
the air may have caused the difference in the results at the two 
periods. ■ 

Finally, the facts just given with respect to Phaseolus 
multifloriLS^ Vicia faba^ and Avena sativa show, as it 
seems to us, that a layer of grease spread for a length of 
1^ to 2 mm. over the tip of the radicle, or the destruction 
of the tip by caustic, greatly lessens or quite annuls in 
the upper and exposed part the power of bending towards 
a neighbouring source of moisture. We should bear in 
mind that the part which bends most, lies at some little 
distance above the greased or cauterised tip; and that 
the rapid growth of this part, proves that it has not been 
injured by the tips having been thus treated. In those 
cases in which the radicles with greased tips became 
curved, it is possible that the layer of grease was not 
sufficiently thick wholly to exclude moisture, or that a 
sufficient length was not thus protected, or, in the case 
of the caustic, not destroyed. When radicles with greased 
tips are left to grow for several days in damp air, the 
grease is drawn out into the finest reticulated threads and 
dots, with narrow portions of the surface left clean. 
Such portions would, it is probable, be able to absorb 
moisture, and thus we can account for several of the 
radicles with greased tips having become curved towards 
the sieve after an interval of one or two days. On the 
whole, we may infer that sensitiveness to a difference in 
the amount of moisture in the air on the two sides of a 
radicle resides in the tip, which transmits some influence 
to the upper part, causing it to bend towards the source 
of moisture. Consequently, the movement is the reverse 
of that caused by objects attached to one side of the tip, 
or by a thin slice being cut off, or by being slightly 



186 THE EFFECT OP KILLING OR Chap. IIL 

cauterised. In a future chapter it will be shown that 
sensitiveness to the attraction of gravity likewise resides 
in the tip ; so that it is the tip which excites the adjoin- 
ing parts of a horizontally extended radicle to bend 
towards the centre of the earth. 

Secondary Radicle becomings veeticallt Geo- 

TROPIC BY the destruction OR INJURY OF THE 

Terminal Part of the Primary Radicle. 

Sachs has shown that the lateral or secondary radicles 
of the bean, and probably of other plants, are acted on by 
geotropism in so peculiar a manner, that they grow out 
horizontally or a little inclined downwards ; and he has 
further shown * the interesting fact, that if the end of 
the primary radicle be cut off, one of the nearest second- 
ary radicles changes its nature and grows perpendicu- 
larly downwards, thus replacing the primary radicle. 
We repeated this experiment, and planted beans with 
amputated radicles in friable peat, and saw the result 
described by Sachs ; but generally two or three of the 
secondary radicles grew perpendicularly downwards. We 
also modified the experiment, by pinching young radicles 
a little way above their tips, between the arms of a U- 
shaped piece of thick leaden wire. The part pinched 
was thus flattened, and was afterwards prevented from 
growing thicker. Five radicles had their ends cut off, 
and served as controls or standards. Eight were pinched ; 
of these 2 were pinched too severely and their ends died 
and dropped off ; 2 were not pinched enough and were 
not sensibly affected ; the remaining 4 were pinched suf- 
ficiently to check the growth of the terminal part, but 
did not appear otherwise injured. When the U-shaped 
wires were removed, after an interval of 15 days, the part 



*• ' Arbeiten Bot. Institut., Wiirzburg,' Heft iv. 1874, p. 622. 



Chap. III. INJUHING THE PRIMARY RADICLE. 187 

beneath the wire was found to be very thin and easily 
broken, whilst the part above was thickened. Now in 
these four cases, one or more of the secondary radicles, 
arising from the thickened part just above the wire, had 
grown perpendicularly downwards. In the best case the 
primary radicle (the part below the wire being 1^ inch 
in length, was somewhat distorted, and was not half as 
long as three adjoining secondary radicles, which had 
grown vertically, or almost vertically, downwards. Some 
of these secondary radicles adhered together or had be- 
come confluent. We learn from these four cases that it 
is not necessary, in order that a secondary radicle should 
assume the nature of a primary one, that the latter should 
be actually amputated ; it is sufficient that the flow of the 
sap into it should be checked, and consequently should be 
directed into the adjoining secondary radicles ; for this 
seems to be the most obvious result of the primary radicle 
being pinched between the arms of a U-shaped wire. 

This change in the nature of secondary radicles is 
clearly analogous, as Sachs has remarked, to that which 
occurs with the shoots of trees, when the leading one is 
destroyed and is afterwards replaced by one or more of 
the lateral shoots ; for these now grow upright instead 
of sub-horizontally. But in this latter case the lateral 
shoots are rendered apogeotropic, whereas with radicles 
the lateral ones are rendered geotropic. We are natu- 
rally led to suspect that the same cause acts with shoots 
as with roots, namely, an increased flow of sap into the 
lateral ones. We made some trials with Abies communis 
and pedinata, by pinching with wire the leading and all 
the lateral shoots excepting one. But we believe that 
they were too old when experimented on ; and some 
were pinched too severely, and some not enough. Only 
one case succeeded, namely with the spruce-fir. The 
leading shoot was not killed, but its growth was checked ; 



188 THE EFFECT OF KILLINGr OR Chap. III. 

at its base there were three lateral shoots in a whorl, two 
of which were pinched, one being thus killed ; the third 
was left untouched. These lateral shoots, when operated 
on (July 14th) stood at an angle of 8° above the horizon ; 
by Sept. 8th the unpinched one had risen 35° ; by Oct. 
4th it had risen 46°, and by Jan. 26th 48°, and it had now 
become a little curved inwards. Part of this rise of 48° 
may be attributed to ordinary growth, for the pinched 
shoot rose 12° within the same period. It thus follows 
that the unpinched shoot stood, on Jan. 26th, 56° above 
the horizon, or 34° from the vertical ; and it was thus 
obviously almost ready to replace the slowly growing, 
pinched, leading shoot. Nevertheless, we feel some 
doubt about this experiment, for we have since observed 
with spruce-firs growing rather unhealthily, that the lat- 
eral shoots near the summit sometimes become highly 
inclined, whilst the leading shoot remains apparently 
sound. 

A widely different agency not rarely causes shoots 
which naturally would have grown out horizontally to 
grow up vertically. The lateral branches of the Silver 
Fir (A. pectinata) are often affected by a fungus, JEci- 
dium eJatinum^ which causes the branch to enlarge into 
an oval knob formed of hard wood, in one of which we 
counted 24 rings of growth. According to De Bary,* 
when the mycelium penetrates a bud beginning to elon- 
gate, the shoot developed from it grows vertically up- 
wards. Such upright shoots afterwards produce lateral 
and horizontal branches ; and they then present a curi- 
ous appearance, as if a young fir-tree had grown out of 
a ball of clay surrounding the branch. These upright 
shoots have manifestly changed their nature and become 



*" See his valuable article in are called in German "Hexen- 
*Rot. Zcitunq;,' 1867, p. 257, on besen," or "witch-brooms." 
tlu'se monstrous growths which 



Chap. HI. INJURING THE PRIMARY RADICLE. 189 

apogeotropic ; for if they had not been affected by the 
^cidium, they would have -grown out horizontally like 
all the other twigs on the same branches. This change 
can hardly be due to an increased flow of sap into the 
part ; but the presence of the mycelium will have greatly 
disturbed its natural constitution. 

According to Mr. Meehan,* the stems of three species 
of Euphorbia and of Portulaca oleracea are " normally 
prostrate or procumbent " ; but when they are attacked 
by an ^cidium, they " assume an erect habit." Dr. Stahl 
informs us that he knows of several analogous cases ; and 
these seem to be closely related to that of the Abies. 
The rhizomes of Sparganium ramosum grow out hori- 
zontally in the soil to a considerable length, or are dia- 
geotropic ; but F. Elf ving found that when they were 
cultivated in water their tips turned upwards, and they 
became apogeotropic. The same result followed when 
the stem of the plant was bent until it cracked or was 
merely much bowed. f 

No explanation has hitherto been attempted of such 
cases as the foregoing, — namely, of secondary radicles 
growing vertically downwards, and of lateral shoots grow- 
ing vertically upwards, after the amputation of the pri- 
mary radicle or of the leading shoot. The following 
considerations give us, as we believe, the clue. Firstly, 
any cause which disturbs the constitution J is apt to 

*■ ' Proc. Acad. Nat. Sc. Phila- J The facts on which the fol- 

delphia,' June 16th, 1874, and lowing conclusions are founded 

July 23rd, 1875. are given in ' The Variation of 

tSee F. Elfving's interesting Animals and Plants under Domes- 
paper in 'Arbeiten Bot. Institut., tication,' 2nd edit. 1875. On the 
in Wiirzburg,' vol. ii. 1880, p. 489. causes leading to reversion see 
Carl Kraus (Triesdorf) had pre- chap. xii. vol. ii. and p. 59, chap, 
viously observed ('Flora,' 1878, xiv. On peloric flowers, chap, 
p. 324) that the underground xiii. p. 32 ; and see p. 337 on their 
shoots of Triticum repens bend position on the plant. With re- 
vertically up when the parts above spect to seeds, p. 340. On rever- 
ground are removed, and when sion by means of buds, p. 438 
the rhizomes are kept partly im- chap. xi. vol. i. 
mersed in water. 



190 SUMMARY OP CHAPTER. Chap. III. 

induce reversion ; such as the crossing of two distinct 
races, or a change of conditions, as when domestic ani- 
mals become feral. But the case which most concerns 
us, is the frequent appearance of peloric flowers on the 
summit of a stem, or in the centre of the inflorescence, 
— parts which, it is believed, receive the most sap ; for 
when an irregular flower becomes perfectly regular or 
peloric, this may be attributed, at least partly, to rever- 
sion to a primitive and normal type. Even the position 
of a seed at the end of a capsule sometimes gives to the 
seedling developed from it a tendency to revert. Sec- 
ondly, reversions often occur by means of buds, inde- 
pendently of reproduction by seed ; so that a bud may 
revert to the character of a former state many bud-gen- 
erations ago. In the case of animals, reversions may 
occur in the individual with advancing age. Thirdly 
and lastly, radicles when they first protrude from the 
seed are always geotropic, and plumules or shoots almost 
always apogeotropic. If then any cause, such as an in- 
creased flow of sap or the presence of mycelium, disturbs 
the constitution of a lateral shoot or of a secondary radi- 
cle, it is apt to revert to its primordial, state; and it 
becomes either apogeotropic or geotropic, as the case may 
be, and consequently grows either vertically upwards or 
downwards. It is indeed possible, or even probable, that 
this tendency to reversion may have been increased, as it 
is manifestly of service to the plant. 

Summary of Chapter. 

A part or organ may be called sensitive, when its 
irritation excites movement in an adjoining part. Now 
it has been shown in this chapter, that the tip of the 
radicle of the bean is in this sense sensitive to the con- 
tact of any small object attached to one side by shellac 
or gum-water ; also to a slight touch with dry caustic, 



Chap. III. SUMMARY OF CHAPTER. 191 

and to a thin slice cut off one side. The radicles of the 
pea were tried with attached objects and caustic, both 
of which acted. With Phaseolus muUifiorus the tip was 
hardly sensitive to small squares of attached card, but 
was sensitive to caustic and to slicing. The radicles of 
Tropaeolum were highly sensitive to contact ; and so, as 
far as we could judge, were those of Gosaypiiim her- 
haceum^ and they were certainly sensitive to caustic. 
The tips of the radicles of Cuciirhita ovifera were like- 
wise highly sensitive to caustic, though only moderately 
So to contact. Raphanus sativus offered a somewhat 
doubtful case. With ^sculus the tips were quite indif- 
ferent to bodies attached to them, though sensitive to 
caustic. Those of Quercus rohur and Zea mays were 
highly sensitive to contact, as were the radicles of the 
latter to caustic. In several of these cases the difference 
in sensitiveness of the tip to contact and to caustic was, 
as we believe, merely apparent; for with Gossypium, 
Eaphanus, and Cucurbita, the tip was so fine and flexible 
that it was very difficult to attach any object to one of its 
sides. With the radicles of ^sculus, the tips were not at 
all sensitive to small bodies attached to them ; but it does 
not follow from this fact that they would not have been 
sensitive to somewhat greater continued pressure, if this 
could have been applied. 

The peculiar form of sensitiveness which we are here 
considering, is confined to the tip of the radicle for a 
length of from 1 mm. to 1*5 mm. When this part is 
irritated by contact with any object, by caustic, or by a 
thin slice being cut off, the upper adjoining part of the 
radicle, for a length of from 6 or 7 to even 12 mm., is 
excited to bend away from the side which has been irri- 
tated. Some influence must therefore be transmitted 
from the tip along the radicle for this length. The cur- 
vature thus caused is generally symmetrical. The part 



192 SUMMARY OF CHAPTER. Chap. III. 

which bends most apparently coincides with that of 
tlie most rapid growth. The tip and. the basal part 
grow very slowly and they bend very little. 

Considering the widely separated, position in the 
vegetable series of the several above-named genera, we 
may conclude that the tips of the radicles of all, or almost 
all, plants are similarly sensitive, and transmit an influ- 
ence causing the upper part to bend. With respect to 
the tips of the secondary radicles, those of Vicia faba, 
Pisum sativum^ and Zea mays were alone observed, and 
they were found similarly sensitive. 

In order that these movements should be properly 
displayed, it appears necessary that the radicles should 
grow at their normal rate. If subjected to a high tem- 
perature and made to grow rapidly, the tips seem either 
to lose their sensitiveness, or the upper part to lose the 
power of bending. So it appears to be if they grow 
very slowly from not being vigorous, or from being kept 
at too low a temperature ; also when they are forced to 
germinate in the middle of the winter. 

The curvature of the radicle sometimes occurs within 
from 6 to 8 hours after the tip has been irritated, and 
almost always within 24 h., excepting in the case of the 
massive radicles of ^sculus. The curvature often 
amounts to a rectangle, — that is, the terminal part bends 
upwards until the tip, which is but little curved, projects 
almost horizontally. Occasionally the tip, from the con- 
tinued irritation of the attached object, continues to bend 
up until it forms a hook with the point directed towards 
the zenith, or a loop, or even a spire. After a time the 
radicle apparently becomes accustomed to the irritation, 
as occurs in the case of tendrils, for it again grows down- 
wards, although the bit of card or other object may 
remain attached to the tip. 

It is evident that a small object attached to the free 



Chap. III. SUMMARY OP CHAPTER. 193 

point of a vertically suspended radicle can offer no 
mechanical resistance to its growth as a whole, for the 
object is carried downwards as the radicle elongates, or 
upwards as the radicle curves upwards. Nor can the 
growth of the tip itself be mechanically checked by an 
object attached to it by gum- water, which remains all 
'the time perfectly soft. The weight of the object, 
though quite insignificant, is opposed to the upward 
curvature. We may therefore conclude that it is the 
irritation due to contact which excites the movement. 
The contact, however, must be prolonged, for the tips 
of 15 radicles were rubbed for a. short time, and this 
did not cause them to bend. Here then we have a 
case of specialised sensibility, like that of the glands 
of Drosera; for these are exquisitely sensitive to the 
slightest pressure if prolonged, but not to two or three 
rough touches. 

When the tip of a radicle is lightly touched on one 
side with dry nitrate of silver, the injury caused is very 
slight, and the adjoining upper part bends away from 
the cauterised point with more certainty in most cases 
than from an object attached on one side. Here it obvi- 
ously is not the mere touch, but the effect produced by 
the caustic, which induces the tip to transmit some influ- 
ence to the adjoining part, causing it to bend away. If 
one side of the tip is badly injured or killed by the 
caustic, it ceases to grow, whilst the opposite side con- 
tinues growing ; and the result is that the tip itself bends 
towards the injured side and often becomes completely 
hooked ; and it is remarkable that in this case the adjoin- 
ing upper part does not bend. The stimulus is too pow- 
erful or the shock too great for the proper influence to 
be transmitted from the tip. We have strictly analogous 
cases with Drosera, Dionsea and Pinguicula, with which 
plants a too powerful stimulus does not excite the tenta- 



194 SUMMARY OF CHAPTER. Chap. III. 

cles to become incurved, or the lobes to close, or the mar- 
gin to be folded inwards. 

With respect to the degree of sensitiveness of the 
apex to contact under favourable conditions, we have 
seen that with Vicia faba a little square of writing-paper 
affixed with shellac sufficed to cause movement ; as did 
on one occasion a square of merely damped goldbeaters' 
skin, but it acted very slowly. Short bits of moderately 
thick bristle (of which measurements have been given) 
affixed with gum-water acted in only three out of eleven 
trials, and beads of dried shellace under -^^^ of a grain 
in weight acted only twice in nine cases ; so that here we 
have nearly reached the minimum of necessary irritation. 
The apex, therefore, is much less sensitive to pressure 
than the glands of Drosera, for these are affected by far 
thinner objects than bits of bristle, and by a very much 
less weight than g-J-g- th of a grain. But the most inter- 
esting evidence of the delicate sensitiveness of the tip of 
the radicle was afforded by its power of discriminating 
between equal-sized squares of card-like and very thin 
paper, when these were attached on opposite sides, as was 
observed with the radicles of the bean and oak. 

When radicles of the bean are extended horizontally 
with squares of card attached to the lower sides of their 
tips, the irritation thus caused was always conquered by 
geotropism, which then acts under the most favourable 
conditions at right angles to the radicle. But when 
objects were attached to the radicles of any of the 
above-named genera, suspended vertically, the irritation 
conquered geotropism, which latter power at first acted 
obliquely on the radicle ; so that the immediate irritation 
from the attached object, aided by its after-effects, pre- 
vailed and caused the radicle to bend upwards, until 
sometimes the point was directed to the zenith. We 
must, however, assume that the after-effects of the irri- 



Chap. III. SUMMARY OF CHAPTER. I95 

tation of the tip by an attached object come into play, 
only after movement has been excited. The tips of the 
radicles of the pea seem to be more sensitive to contact 
than those of the bean, for when they were extended 
horizontally with squares of card adhering to their lower 
sides, a most curious struggle occasionally arose, some- 
times one and sometimes the other force prevailing, but 
ultimately geotropism was always victorious ; neverthe- 
less, in two instances the terminal part became so much 
curved upwards that loops were subsequently formed. 
With the pea, therefore, the irritation from an attached 
object, and from geotropism when acting at right angles 
to the radicle, are nearly balanced forces. Closely simi- 
lar results were observed with the horizontally extended 
radicles of Cucurbita ovifera, when their tips were 
slightly cauterised on the lower side. 

Finally, the several co-ordinated movements by which 
radicles are enabled to perform their proper functions 
are admirably perfect. In whatever direction the pri- 
mary radicle first protrudes from the seed, geotropism 
guides it perpendicularly downwards ; and the capacity 
to be acted on by the attraction of gravity resides in the 
tip. But Sachs has proved* that the secondary radi- 
cles, or those emitted by the primary one, are acted on 
by geotropism in such a manner that they tend to bend 
only obliquely downwards. If they had been acted on 
like the primary radicle, all the radicles would have pene- 
trated the ground in a close bundle. We have seen that 
if the end of the primary radicle is cut off or injured, 
the adjoining secondary radicles become geotropic and 
grow vertically downwards. This power must often be 
of great service to the plant, when the primary radicle 
has been destroyed by the larvae of insects, burrowing 



'*• 'ArbeitenBot. Institute Wurzburg,' Ileft iv. 1874, pp. G05-631. 



196 SUMMARY OF CHAPTER. Chap. III. 

animals, or any other accident. The tertiary radicles, or 
those emitted by the secondary ones, are not influenced, 
at least in the case of the bean, by geotropism ; so they 
grow out freely in all directions. From this manner of 
growth of the various kinds of radicles, they are distrib- 
uted, together with their absorbent hairs, throughout the 
surrounding soil, as Sachs has remarked, in the most 
advantageous manner ; for the whole soil is thus closely 
searched. 

Geotropism, as was shown in the last chapter, excites 
the primary radicle to bend downwards with very little 
force, quite insufficient to penetrate the ground. Such 
penetration is effected by the pointed apex (protected 
by the root-cap) being pressed down by the longitudinal 
expansion or growth of the terminal rigid portion, aided 
by its transverse expansion, both of which forces act 
powerfully. It is, however, indispensable that the seeds 
should be at first held down in some manner. When 
they lie on the bare surface they are held down by the 
attachment of the root-hairs to any adjoining objects ; 
and this apparently is efi'ected by the conversion of their 
outer surface into a cement. But many seeds get cov- 
ered up by various accidents, or they fall into crevices or 
holes. With some seeds their own weight suffices. 

The circumnutating movement of the terminal grow- 
ing part both of the primary and secondary radicles is so 
feeble that it can aid them very little in penetrating the 
ground, excepting when the superficial layer is very soft 
and damp. But it must aid them materially when they 
happen to break obliquely into cracks, or into burrows 
made by earth-worms or larvoe. This movement, more- 
over, combined with the sensitiveness of the tip to con- 
tact, can hardly fail to be of the highest importance ; for 
as the tip is always endeavouring to bend to all sides it 
will i)ress on all sides, and will thus be able to discrim- 



Chap. III. SUMMARY OP CHAPTER. 197 

inate between the harder and softer adjoining surfaces, 
in the same manner as it discriminated between the at- 
tached squares of card-like and thin paper. Conse- 
quently it will tend to bend from the harder soil, and 
will thus follow the lines of least resistance. So it will 
be if it meets with a stone or the root of another plant in 
the soil, as must incessantly occur. If the tip were not 
sensitive, and if it did not excite the upper part of the 
root to bend away, whenever it encountered at right 
angles some obstacle in the ground, it would be liable to 
be doubled up into a contorted mass. But we have seen 
with radicles growing down inclined plates of glass, that 
as soon as the tip merely touched a slip of wood cemented 
across the plate, the whole terminal growing part curved 
away, so that the tip soon stood at right angles to its for- 
mer direction ; and thus it would be with an obstacle en- 
countered in the ground, as far as the pressure of the 
surrounding soil would permit. We can also uaderstand 
why thick and strong radicles, like those of ^sculus, 
should be endowed with less sensitiveness than more 
delicate ones ; for the former would be able by the force 
of their growth to overcome any slight obstacle. 

After a radicle, which has been deflected by some 
stone or root from its natural downward course, reaches 
the edge of the obstacle, geotropism will direct it to 
grow again straight downward ; but we know that ge- 
otropism acts with very little force, and here another 
excellent adaptation, as Sachs has remarked,* comes into 
play. For the upper part of the radicle, a little above 
the apex, is, as we have seen, likewise sensitive ; and this 
sensitiveness causes the radicle to bend like a tendril 
towards the touching object, so that as it rubs over the 
edge of an obstacle, it will bend downwards ; and the 



■* ' Arbeiten Bot. Inst. Wiirzburg,' Heft iii. p. 45G. 



198 SUMMARY OF CHAPTER. Chap. III. 

curvature thus induced is abrupt, in which respect it 
differs from that caused by the irritation of one side of 
the tip. This downward bending coincides with that 
due to geotropism, and both will cause the root to re- 
sume its original course. 

As radicles perceive an excess of moisture in the air 
on one side and bend towards this side, we may infer 
that they will act in the same manner with respect to 
moisture in the earth. The sensitiveness to moisture 
resides in the tip, which determines the bending of the 
upper part. This capacity perhaps partly accounts for 
the extent to which drain - pipes often become choked 
with roots. 

Considering the several facts given in this chapter, 
we see that the course followed by a root through the 
soil is governed by extraordinarily complex and diversi- 
fied agencies, — by geotropism acting in a different man- 
ner on the primary, secondary, and tertiary radicles, — by 
sensitiveness to contact, different in kind in the apex and 
in the part immediately above the apex, and apparently 
by sensitiveness to the varying dampness of different parts 
of the soil. These several stimuli to movement are all 
more powerful than geotropism, when this acts obliquely 
on a radicle, which has been deflected from its perpen- 
dicular downward course. The roots, moreover, of most 
plants are excited by light to bend either to or from it ; 
but as roots are not naturally exposed to the light it is 
doubtful whether this sensitiveness, which is perhaps 
only the indirect result of the radicles being highly sen- 
sitive to other stimuli, is of any service to the plant. 
The direction which the apex takes at each successive 
period of the growth of a root, ultimately determines its 
whole course ; it is therefore highly important that the 
apex should pursue from the first the most advantageous 
direction ; and we can thus understand why sensitiveness 



Chap. III. SUMMARY OF CHAPTER. I99 

to geotropism, to contact and to moisture, all reside in 
the tip, and why the tip determines the upper growing 
part to bend either from or to the exciting cause. A 
radicle may be compared with a burrowing animal such 
as a mole, which wishes to penetrate perpendicularly 
down into the ground. By continually moving his head 
from side to side, or circumnutating, he will feel any 
stone or other obstacle, as well as any difference in the 
hardness of the soil, and he will turn from that side ; 
if the earth is damper on one than on the other side 
he will turn thitherward as a better hunting-ground. 
Nevertheless, after each interruption, guided by the 
sense of gravity, he will be able to recover his down- 
ward course and to burrow to a greater depth. 



14 



CHAPTER IV. 

The Circumnutating Movements of the several parts 
OP Mature Plants. 

Circumnutation of steins : concluding remarks on — Circumnutation of 
stolons : aid thus afforded to winding amongst the stems of sur- 
rounding plants — Circumnutation of flower-stems — Circumnuta- 
tion of Dicotyledonous leaves — Singular oscillatory movement of 

- , leaves of Dionsea — Leaves of Cannabis sink at night — Leaves of 
Gymnosperms — Of Monocotyledons — Cryptogams — Concluding re- 
marks on the circumnutation of leaves : generally rise in the 
evening and sink in the morning. 

We have seen in the first chapter that the stems of all 
seedlings, whether hypocotyls or epicotjls, as well as the 
cotyledons and the radicles, are continually circumnu- 
tating — that is, they grow first on one side and then on 
another, such growth being probably preceded by in- 
creased turgescence of the cells. As it was unlikely that 
plants should change their manner of growth with ad- 
vancing age, it seemed probable that the various organs 
of all plants at all ages, as long as they continued to grow, 
would be found to circumnutate, though perhaps to an 
extremely small extent. As it was important for us to 
discover whether this was the case, we determined to 
observe carefully a certain number of plants which were 
growing vigorously, and which were not known to move 
in any manner. We commenced with stems. Observa- 
tions of this kind are tedious, and it appeared to us that 
it would be sufficient to observe the stems in about a 
score of genera, belonging to widely distinct families and 
inhabitants of various countries. Several plants were 
200 



Chap. IV. CIRCUMNUTATION OF STEMS. 201 

selected which, from being woody, or for other reasons, 
seemed the least likely to circumnutate. The observa- 
tions and the diagrams were made in the manner de- 
scribed in the Introduction. Plants in pots were sub- 
jected to a proper temperature, and whilst being observed, 
were kept either in darkness or were feebly illuminated 
from above. They are arranged, in the order adopted by 
Hooker in Le Maout and Decaisne's ' System of Botany.' 
The number of the family to which each genus belongs 
is appended, as this serves to show the place of each in 
the series. 

(1.) Ileris umbellata (Cruciferss, Fam. 14). — The movement 
of the stem of a young plant, 4 inches in height, consisting of 
four internodes (the hypocotyl included) besides a large bud on 
the summit, was traced, as here shown, during 34 h. (Fig. 70j. 



Fig. 70. 




Iberis umbellata : circumnutation of stem of young plant, traced from 
8.30 A.M. Sept. 13th to same hour on following morning. Distance 
of summit of stem beneath the horizontal glass 7 "6 inches. Dia- 
gram reduced to half of original size. Movement as here shown 
magnified between 4 and 5 times. 

As far as we could judge the uppermost inch alone of the stem 
circumnutated, and this in a simple manner. The movement 
was slow, and the rate very unequal at different times. In 
part of its course an irregular ellipse, or rather triangle, was 
completed in 6 h. 30 m. 

(2.) Brassica oleracea (Cruciferae). — A very young plant, bear- 
ing three leaves, of which the longest was only three-quarters 
of an inch in length, was placed under a microscope, furnished 
with an eye-piece micrometer and the tip of the largest leaf was 



202 CIRCUMNUTATION OF STEMS. Chap. IV. 

found to be in constant movement. It crossed five divisions of 
the micrometer, that is, yj o*h of an inch, in 6 m. 20 s. There 
could hardly be a doubt that it was the stem which chiefly moved, 
for the tip did not get quickly out of focus; and this would 
have occurred had the movement been confined to the leaf, 
which moves up or down in nearly the same vertical plane. 

(3.) Linum usitaUssimum {Linese, Fam. 39). — The stems of 
this plant, shortly before the flowering period, are stated by 
Fritz Mtiller (' Jenaische Zeitschrif t, ' B. v. p. 137) to revolve, 
or circumnutate. 

(4.) Pelargonium zonale (Geraniacese, Fam. 47). — A young 
plant, 7| inches in height, was observed in the usual manner; 
but, in order to see the bead at the end of the glass filament 
and at the same time the mark beneath, it was necessary to cut 



Fig. 71. 



£0°20p.m.9'^ 

SCa.m.lO'f) 







Pelargonium zonale: circumnutation of stem of young plant, feebly 
illuminated from above. Movement of bead magnified about 11 
times ; traced on a horizontal glass from noon on March 9tli to 
8 A.M. on the 11th. 

off three leaves on one side. We do not know whether it was 
owing to this cause, or to the plant having previously become 
bent to one side through heliotropism, but from the morning of 
the 7th of March to 10.30 p.m. on the 8th, the stem moved a 
considerable distance in a zigzag line, in the same general di- 
rection. During the night of the 8th it moved to some dis- 
tance at right angles to its former course, and next morning 
(9th) stood for a time almost still. At noon on the 9th a new 
tracing was begun (see Fig. 71), which was continued till 8 a.m. 
OTi the 11th. Between noon on the 9th and 5 p.m. on the 10th 
(i.e. in the course of 29 h.), the stem described a circle. This 
plant therefore circumuutates, but at a very slow rate, and to a 
small extent. 



Chap. IV. CIRCUMNUTATION OF STEMS. 



203 



(5.) TropoBolum majus (?) (dwarfed var. called Tom Thumb); 
(Geraniacese, Fam. 47). — The species of this genus climb by the 
aid of their sensitive petioles, but some of them also twine 



Fig. 72. 




Tropxolum majus (f) : circumnutation of stem of young plant, traced 
on a horizontal glass from 9 A.M. Dec. 26th to 10 a.m. on 27th. 
Movement of bead magnified about 5 times, and here reduced to 
half of original scale. 



round supports; but even these latter species do not begin ta 

circumnutate in a conspicuous manner whilst young. The 

variety here treated of has a rather thick stem, and is so dwarf 

that apparently it does _ 

^ r u- Fig. 73. 

not climb m any manner. 

We therefore wished to 
ascertain whether the 
stem of a young plant, 
consisting of two in- 
ternodes, together 3-2 
inches in height, circum- 
nutated. It was ob- 
served during 25 h., and 
we see in Fig. 72 that 
the stem moved in a zig- 
zag course, indicating 
circumnutation. 

(6.) Trifolium resu- 
pinatum (Leguminosse, 
Fam. 75). — When we 
treat of the sleep of 
plants, we shall see that 
the stems in several Le- 
guminous genera, for instance, those of Hedysarum, Mimosa, 
Melilotus, &c., which are not climbers, circumnutate in a con- 




Trifolium resupinatum : circumnutation of 
stem, traced on vertical glass from 9.30 
A.M. to 4.30 P.M. Nov. 3rd. Tracing not 
greatly magnified, reduced to half of 
original size. Plant feebly illuminated 
from above. 



204 



CIRCUMNUTATION OF STEMS. Chap. IV. 



spicuous manner. We will here give only a single instance 
(Fig. 73), showing the circumnutation of the stem of a large 

Fig. 74. 




Fig. 75. 



Ruhus (hyboid) : circumnutation of stem, traced on horizontal glass, 
from 4 P.M. March 14th to 8.30 A.M. 16th. Tracing much magni- 
fied, reduced to half of original size. Plant illuminated feebly 
from above. 

plant of a clover, Trifolium resupinatum. In the course of 7 h. 

the stem changed its course greatly eight times and completed 
three irregular circles or ellipses. 
It therefore circumnutated rapidly. 
Some of the lines run at right angles 
to one another. 

(7.) Subus idceus (hybrid) (Rosa- 
ceae, Fam. 76). — As we happened to 
have a young plant, 11 inches in 
height and growing vigorously, 
which had been raised from a cross 
between the raspberry {Rubus idceus) 
and a North American Rubus, it was 
observed in the usual manner. Dur- 
ing the morning of March 14th the 
stem almost completed a circle, and 
then moved far to the right. At 4 
P.M. it reversed its course, and now 
a fresh tracing was begun, which was 
continued during 40^ h., and is given 
in Fig. 74. We here have well- 
marked circumnutation. 

(8.) Deutzia gracilis (Saxifragefe, 
Fam. 77). — A shoot on a bush about 
18 inches in height was observed. 




Deutzia gracilis : circumnu- 
tation of stem, kept in 
darkness, traced on hori- 
zontal glass, from 8 30 
A.M. to 7 P.M. March 20th. 
Movement of bead origi- 
nally magnified about 20 
times, lucre reduced to 
half scale. 



Chap. IV. CIRCUMNUTATION OF STEMS. 205 

The bead changed its course greatly eleven times in the course 
of 10 h. 30 m. (Fig. 75), and there could be no doubt about 
the circumnutation of the stem. 

(9.) Fuchsia (greenhouse var,, with large flowers, probably 
a hybrid) (Onagrarieae, Fam. 100). — A young plant, 15 inches 
in height, was observed during nearly 48 h. The accompany- 
ing figure (Fig. 76) gives the necessary particulars, and shows 
that the stem circumnutated, though rather slowly. 




Fuchsia (garden var.) : circumnutation of stem, kept in darkness, 
traced on horizontal glass, from 8.30 a.m. to 7 p.m. March 20th. 
Movement of bead originally magnified about 40 times, here re- . 
duced to half scale. 

(10.) Cereus speciocissimus (garden var., sometimes called 
Phyllocactus multiflorus) (Cactese, Fam. 109). — This plant 
which was growing vigorously from having been removed a few 
days before from the greenhouse to the hot-house, was observed 
with especial interest, as it seemed so little probable that the 
stem would circumnutate. The branches are flat, or flabelli- 
form ; but some of them are triangular in section, with the three 
sides hollowed out. A branch of this latter shape, 9 inches in 
length and H in diameter, was chosen for observation, as less 
likely to circumnutate than a flabelliform branch. The move- 
ment of the bead at the end of the glass filament, aflfixed to the 
summit of the branch, was traced (A, Fig. 77) from 9.23 a.m. 



206 



CIRCUMNUTATION OF STEMS. 



Chap. IV. 



to 4.30 P.M. on Nov. 23rd, during which time it changed its 
course greatly six times. On the 34th another tracing was made 
(see B), and the bead on this day changed its course oftener, 



Fig. 77. 



S'SO'a 




4^30'p.m, 



fcLtn.S5\ 



9' 2 3' cum, 




.Sd^ 



Cereus speciocissimus : circumnutation of stem, illuniinated from above, 
traced on a horizontal glass, in A from 9 A.M. to 4.30 p.m. on Nov. 
23rd ; and in B from 8.30 A.M. on the 24th to 8 a.m. on the 25th. 
Movement of the bead in B magnified about 38 times. 



making in 8 h. what may be considered as four ellipses, with 
their longer axes differently directed. The position of the stem 
and its commencing course on the following morning are like- 
wise shown. There can be no doubt that this branch, though, 
appearing quite rigid, circumnutated ; but the extreme amount 
of movement during the time was very small, probably rather 
less than the ^V^h of an inch. 

(11.) Hedera helix (Araliacese, Fam. 114). — The stem is 
known to be apheliotropic, and several seedlings growing in a 
pot in the greenhouse became bent in the middle of the summer 
at right angles from the light. On Sept. 2nd some of these 
stems were tied up so as to stand vertically, and were placed be- 
fore a north-east window ; but to our surprise they were now 
decidedly heliotropic, for during 4 days they curved themselves 



Chap. IV. CIRCUMNUTATION OP STEMS. 207 

towards the light, and their course being traced on a horizontal 
glass, was strongly zigzag. During the 6 succeeding days they 
circumnutated over the same small space at a slow rate, but there 
could be no doubt about their circumnutation. The plants were 
kept exactly in the same place before the window, and after an 
interval of 15 days the stems were again observed during 2 days 
and their movements traced, and they were found to be still cir- 
cumnutating, but on a yet smaller scale. 

(12.) Gazania ringens (Composit^e, Fam. 123). — The circum- 
nutation of the stem of a young plant, 7 inches in height, as 
measured to the tip of the highest leaf, was traced during 33 h., 
and is shown in the accompanying figure (Fig. 78). Two main 
lines may be observed running at nearly right angles to two 
other main lines ; but these are interrupted by small loops. 



Fig. 78. 



etjim.8. 




S^(mL22^'^ 



il0°3fpm.2i^ 



Gazania ringens: circumnutation of stem traced from 9 A.M. March 
21st to 6 P.M. on 22nd ; plant kept in darkness. Movement of 
bead at the close of the observations magnified 34 times, here re- 
duced to half the original scale. 

(13.) Azalea Tndica (Ericineae, Fam. 128). — A bush 21 inches 
in height was selected for observation, and the circumnutation 
of its leading shoot was traced during 26 h. 40 m., as shown in 
the following figure (Fig. 79). 

(14.) Plumhaqo Capenais (Plumbagine^i, Fam. 134). — A small 
lateral branch which projected from a tall freely growing bush, 
at an angle of 35" above the horizon, was selected for observa- 
tion. For the first 11 h. it moved to a considerable distance in 
a nearly straight line to one side, owing probably to its having 



208 



CIRCUMNUTATION OF STEMS. Chap. IV. 



been previously deflected by the light whilst standing in the 
greenhouse. At 7.20 p.m. on March 7th a fresh tracing was be- 
gun and continued for the next 43 h. 40 m. (see Fig. 80). Dur- 
ing the first 2 h. it followed nearly the same direction as before, 
and then changed it a little ; during the night it mov^d at nearly 
right angles to its previous course. Next day (8th) it zigzagged 




Azalea Indica : circumnutation 
of stem, inuminated from 
above, traced on horizontal 
glass, from 9.30 a.m. March 
9th, to 12.10 P.M. on the 10th. 
But on the morning of the 
10th only four dots were made 
between 8.30 a.m. and 12.10 
P.M., both hours included, so 
that the circumnutation is 
not fairly represented in this 
part of the diagram. Move- 
ment of the bead here mag- 
nified about 30 times. 




Plumbago Capensis : circumnuta- 
tion of tip of a lateral branch, 
traced on horizontal glass, 
from 7.20 p.m. on ]\rarch 7th 
to 3 P.M. on the 9th. Move- 
ment of bead magnified 13 
times. Plant feebly illumi- 
nated from above. 



greatly, and on the 9th moved irregularly round and round a 
small circular space. By 3 p.m. on the 9th the figure had be- 
come so complicated that no more dots could be made ; but the 
shoot continued during the evening of the 9th, the whole of the 
10th, and the morning of the 11th to circumnutate over the 
same small space, which was only about the ^^th of an inch (-97 
mm.) in diameter. Although this branch circumnutated to a 



Chap. IV. CIRCUMNUTATION OF STEMS. 



209 



very small extent, yet it changed its course frequently. The 
movements ought to have been more magnified. 

(15.) Aloysia citriodora (Verbenacese, Fam. 173). — The fol- 
lowing figure (Fig. 81) gives the movements of a shoot during 



Fig. 81. 




Aloysia citriodora : circumnutation of stem, traced from 8.20 A.M. on 
March 22nd to 4 p.m. on 23rd. Plant kept in darkness. Move- 
ment magnified about 40 times. 

31 h. 40 m., and shows that it circumnutated. The bush was 
15 inches in height. 

(16.) Verbena melindres (?) (a scarlet-flowered herbaceous 
var.) (Yerbenacese). — A shoot 8 inches in height had been laid 



fJWt 



Fig. 82. 




5'30'fi.m.S^ 



Verbena melindres: circuranutation of stem in darkness, traced on ver- 
tical glass, from 5.30 p.m. on June 5th to 11 A.M. June 7th. Move- 
ment of bead magnified 9 times. 

horizontally, for the sake of observing its apogeotropism, and 
the terminal portion had grown vertically upwards for a length 



210 CIRCUMNUTATION OF STEMS. Chap. IV. 

of 1^ inches. A glass filament, with a bead at the end, was 
fixed upright to the tip, and its movements were traced during 
41 h. 30 m. on a vertical glass (Fig. 82). Under these circum- 
stances the lateral movements were chiefly shown ; but as the 
lines from side to side are not on the same level, the shoot 
must have moved in a plane at right angles to that of the lateral 
movement, that is, it must have circumnutated. On the next 
day (6th) the shoot moved in the course of 16 h. four times to 
the right, and four times to the left ; and this apparently repre- 
sents the formation of four ellipses, so that each was com- 
pleted in 4 h. 

(17.) Ceratopliyllum demersum (Ceratophylleae, Fam, 220). — 
An interesting account of the movements of the stem of this 
water-plant has been published by M. E. Rodier.* The move- 
ments are confined to the young internodes, becoming less and 
less lower down the stem ; and they are extraordinary from their 
amplitude. The stems sometimes moved through an angle of 
above 200° in 6 h., and in one instance through 220° in 3 h. 
They generally bent from right to left in the morning, and in 
an opposite direction in the afternoon; but the movement was 
sometimes temporarily reversed or quite arrested. It was not 
affected by light. It does not appear that M. Rodier made any 
diagram on a horizontal plane representing the actual course 
pursued by the apex, but he speaks of the "branches executing 
round their axes of growth a movement of torsion." From the 
particulars above given, and remembering in the case of twin- 
ing plants and of tendrils, how difficult it is not to mistake their 
bending to all points of the compass for true torsion, we are 
led to believe that the stems of this Ceratophyllum circumnutate, 
probably in the shape of narrow ellipses, each completed in 
about 26 h. The following statement, however, seems to indi- 
cate something different from ordinary circumnutation, but we 
cannot fully understand it. M. Rodier says : "II est alors facile 
de voir que le mouvement de flexion se produit d''dbord dans les 
m6rithalles sup6rieurs, qu'il se propage ensuite, en s'amoindris- 
sant du haut en &as; tandis qu'au contraire le mouvement de 



■■'• 'Comptes Rcndus,' April 30th, lished separately in Bordeaux, 
1877. Also a second notice pub- Nov. 12th, 1877. 



Chap. IV. CIRCUMNUTATION OP STEMS. 



211 



redressement commence par la parti e inf^rieure pour se terminer 
a la partie supcrieure qui, quelquefois, peu de temps avant de 
se relever tout a fait, forme avec I'axe un angle tres aigu." 

(18.) Coniferce. — Dr. Maxwell Masters states (' Journal Linn. 
Soc.,' Dec. 2nd, 1879) that the leading shoots of many Coniferso 
during the season of their active growth exhibit very remark- 
able movements of revolving nutation, that is, they circumnu- 
tate. We may feel sure that the lateral shoots whilst growing 
would exhibit the same movement if carefully observed. 



Fig. 83. 



/^°37|^ A .,8'55'a.w. le^h 



&p.m1S7:- 




fl5[mn. 



S'a.m.U^^ 



Lilinm nuratum : circumnutation of a stem in darkness, traced on a 
horizontal glass, from 8 A.M. on March 14th to 8.35 A.M. on Ifith. 
But it should be noted that our observations were interrupted be- 
tween 6 P.M. on the 14th and 12.15 p.m. on 15th, and the move- 
ments during this interval of 18 h. 15 ra. are represented by a long 
broken line. Diagram reduced to half original scale. 



(19.) Lilium auratum (Fam, Liliacese). — The circumnutation 
of the stem of a plant 34 inches in height is represented in the 
above figure (Fig. 83). 

(20.) Gyperus alternifoliu8 (Fam. Cyperaceae). — A glass fila- 
ment, with a bead at the end, was fixed across the summit of a 
young stem 10 inches in height, close beneath the crown of 
elongated leaves. On March 8th, between 12.20 and 7,20 p.m., 
the stem described an ellipse, open at one end. On the follow- 



212 CIRCUMNUTATION OF STEMS. Chap. IV. 

ing day a new tracing was begun (Fig. 84), which plainly shows 
that the stem completed three irregular figures in the course of 
35 h. 15 m. 

Fig. 84. 




Cyperus alternifoUus : circumnutation of stem, illuminated from above, 
traced on horizontal glass, from 9.45 A.M. March 9th to 9 p.m. on 
10th. The stem grew so rapidly whilst being observed, that it 
was not possible to estimate how much its movements were mag- 
nified in the tracing. 

Concluding Remarlcs on the Circumnutation of Stems. 
— Any one who will inspect the diagrams now given, and 
will bear in mind the widely separated position of the 
plants described in the series, — remembering that we 
have good grounds for the belief that the hypocotyls and 
epicotyls of all seedlings circumnutate, — not forgetting 
the number of plants distributed in the most distinct 
families which climb by a similar movement, — will prob- 
ably admit that the growing stems of all plants, if care- 
fully observed, would be found to circumnutate to a 
greater or less extent. When we treat of the sleep and 
other movements of plants, many other cases of circum- 
nutating stems will be incidentally given. In looking at 
the diagrams, we should remember that the stems were 
always growing, so that in each case the circumnutating 
apex as it rose will have described a spire of some kind. 
The dots were made on the glasses generally at intervals 
of an hour, or hour and a half, and were then joined by 



Chap. IV. CIECUMNUTATION OF STOLONS. 213 

straight lines. If they had been made at intervals of 2 
or 3 minutes, the lines would have been more curvilinear, 
as in the case of the tracks left on the smoked glass- 
plates by the tips of the circumnutating radicles of seed- 
ling plants. The diagrams generally approach in form 
to a succession of more or less irregular ellipses or ovals, 
with their longer axes directed to different points of the 
compass during the same day or on succeediug days. 
The stems therefore, sooner or later, bend to all sides ; 
but after a stem has bent in any one direction, it com- 
monly bends back at first in nearly, though not quite, 
the opposite direction ; and this gives the tendency to 
the formation of ellipses, which are generally narrow, 
but not so narrow as those described by stolons and 
leaves. On the other hand, the figures sometimes ap- 
proach in shape to circles. Whatever the figure may be, 
the course pursued is often interrupted by zigzags, small 
triangles, loops, or ellipses. A stem may describe a sin- 
gle large ellipse one day, and two on the next. With 
different plants the complexity, rate, and amount of 
movement differs much. The stems, for instance, of 
Iberis and Azalea described only a single large ellipse in 
24 h. ; whereas those of the Deutzia made four or five 
deep zigzags or narrow ellipses in 11| h., and those of 
the Trifolium three triangular or quadrilateral figures 
in 7 h. 

CiRCUMis-UTATION- OF STOLON'S OR EUNi^ERS. 

Stolons consist of much elongated, flexible branches, 
which run along the surface of the ground and form 
roots at a distance from the parent-plant. They are 
therefore of the same homological nature as stems ; and 
the three following cases may be added to the twenty 
previously given cases. 



2U 



CIRCUMNUTATION OF STOLONS. Chap. IV. 



Fragaria (cultiv3ited garden var.) : liosacem. — A plant grow- 
ing in a pot had emitted a long stolon ; this was supported by a 
stick, so that it projected for the length of several inches hori- 
zontally. A glass filament bearing two minute triangles of 
paper was aflixed to the terminal bud, which was a little up- 
turned; and its movements were traced during 21 h., as shown 
in Fig. 85. In the course of the first 12 h. it moved twice up 
and twice down in somewhat zigzag lines, and no doubt trav- 
elled in the same manner during the night. On the following 

Fig. 85. 

6''45'amd9*?' 



lO'pm.^ 



XCt4,5^n.7n 




7°45'ajnJ9^ 



Fragaria : circumnutation of stolon, kept in darkness, traced on ver- 
tical glass, from 10.45 a.m. May 18th to 7.45 A.M. on 19tli. 



morning after an interval of 20 h. the apex stood a little higher 
than it did at first, and this shows that the stolon had not been 
acted on within this time by geotropism ; * nor had its own 
weight caused it to bend downwards. 



■*Dr. A. B. Frank states ('Die 
Naturliclie wagerechte Richtung 
von Pflanzcntheilen,' 1870, p. 20) 
that the stolons of this plant are 



acted on by geotropism, but only 
after a considei-able interval of 
time. 



Chap. IV. CIRCUMNUTATION OP STOLONS. 



215 



On the following morning (19th) the glass filament was 
detached and refixed close behind the bud, as it appeared 
possible that the circumnutation of the terminal bud and of 
the adjoining part of the 



stolon might be differ- 
ent. The movement was 
now traced during two 
consecutive days (Fig. 
86). " During the first 
day the filament trav- 
elled in the course of 
14 h. 30 m. five times up 
and four times down, be- 
sides some lateral move- 
ment. On the 20th the 
course was even more 
complicated, and can 
hardly be followed in the 
figure; but the filament 
moved in 16 h. at least 
five times up and five 
times down, with very 
little lateral deflection. 
The first and last dots 
made on this second day, 
viz., at 7 A.M. and 11 
P.M., were close together, 
showing that the stolon 
had not fallen or risen. 
Nevertheless, by com- 
paring its position on the 
morning of the 19th and 



Fig. 86. 



'20'p.mdB^ 




S°a.fn.21^' 



Fragaria: circumnutation of the same 
stolon as in the last figure, observed in 
the same manner, and traced from 
8 A.M. May 19tli to 8 a.m. 21st. 



the stolon had sunk ; and 

this may be attributed to 

slow bending down either from its own weight or from geot- 

ropism. 

During a part of the 20th an orthogonal tracing was made 
by applying a cube of wood to the vertical glass and bringing 
15 



216 CmCUMNUTATION OF STOLONS. Chap. IV. 

the apex of the stolon at successive periods into a line with one 
edge ; a dot being made each time on the glass. This tracing 
therefore represented very nearly the actual amount of move- 
ment of the apex ; and in the course of 9 h. the distance of the 
extreme dots from one another was '45 inch. By the same 
method it was ascertained that the apex moved between 7 a.m. 
on the 20th and 8 a.m. on the 21st a distance of -82 inch. 

A younger and shorter stolon was supported so that it pro- 
jected at about 45° above the horizon, and its movement was 
traced by the same orthogonal method. On the first day the 
apex soon rose above the field of vision. By the next morning 
it had sunk, and the course pursued was now traced during 14 



irio'a.m.19'^ Fig. 87. 




io:itm 



rsopm 



9°a.m. 



Fragaria : circumnutation of another and younger stolon, traced from 
8 A.M. to 10.30 P.M. Figure reduced to one-half of original scale. 

h. 30 ra. (Fig. 87). The amount of movement was almost the 
same, from side to side as up and down ; and differed in this 
respect remarkably from the movement in the previous cases. 
During the latter part of the day, viz., between 3 and 10.30 
P.M., the actual distance travelled by the apex amounted to 1*15 
inch; and in the course of the whole day to at least 2 '67 inch. 
This is an amount of movement almost comparable with that of 
some climbing plants. The same stolon was observed on the 
following day, and now it moved in a somewhat less cornplex 



Chap. IV. CIRCUMNUTATION OF STOLONS. 217 

manner, in a plane not far from vertical. The extreme amount 
of actual movement was 1'55 inch in one direction, and '6 inch 
in another direction at right angles. During neither of these 
days did the stolon bend downwards through geotropism or its 
own weight. 

Four stolons still attached to the plant were laid on damp 
sand in the back of a room, with their tips facing the north-east 
windows. They were thus placed because De Yries says * that 
they are apheiiotropic when exposed to the light of the sun ; but 
we could not perceive any effect from the above feeble degree 
of illumination. We may add that on another occasion, late in 
the summer, some stolons, placed upright before a south-west 
window on a cloudy day, became distinctly curved towards the 
light, and were therefore heliotropic. Close in front of the tips 
of the prostrate stolons, a crowd of very thin sticks and the 
dried haulms of grasses were driven into the sand, to represent 
the crowded stems of surrounding plants in a state of nature. 
This was done for the sake of observing how the growing sto- 
lons would pass through them. They did so easily in the course 
of 6 days, and their circumnutation apparently facilitated their 
passage. When the tips encountered sticks so close together 
that they could not pass between them, they rose up and passed 
over them. The sticks and haulms were removed after the 
passage of the four stolons, two of which were found to have 
assumed a permanently sinuous shape, and two were still straight. 
But to this subject we shall recur under Saxifraga. 

Saxifraga sarmentosa (Saxifragese). — A plant in a suspended 
pot had emitted long branched stolons, which depended like 
threads on all sides. Two were tied up so as to stand vertically, 
and their upper ends became gradually bent downwards, but so 
slowly in the course of several days, that the bending was proba- 
bly due to their weight and not to geotropism. A glass fila- 
ment with little triangles of paper was fixed to the end of one of 
these stolons, which was 17^ inches in length, and had already 
become much bent down, but still projected at a considerable 
angle above the horizon. It moved only slightly three times 
from side to side and then upwards ; on the following day the 



Arbeiten Bot. Inst., Wurzburg,' 1872. p. 434. 



218 CIRCUMNUTATION OF STOLONS. Chap. IV. 

movement was even less. As this stolon was so long we thought 
that its growth was nearly completed, so we tried another which 
was thicker and shorter, viz., lOJ inches in length. It moved 
greatly, chiefly upwards, and changed its course five times in 
the course of the day. During the night it curved so much up- 
wards in opposition to gravity, that the movement could no 
longer be traced on the vertical glass, and a horizontal one had 
to be used. The movement was followed during the next 35 
h., as shown in Fig. 88. Three irregular ellipses, with their 
longer axes somewhat differently directed, were almost com- 
pleted in the first 15 h. The extreme actual amount of move- 
ment of the tip during the 25 h. was -75 inch. 



Fig. 88, 



V 




Saxifraga sarmentosa : circumnutation of an inclined stolon, traced in 
darkness on a horizontal glass, from 7.45 a.m. April 18th to 9 A.M. 
on 9th. Movement of end of stolon magnified 2 "2 times. 

Several stolons were laid on a flat surface of damp sand, in 
the same manner as with those of the strawberry. The friction 
of the sand did not interfere with their circumnutation; nor 
could we detect any evidence of their being sensitive to contact. 
In order to see how in a state of nature they would act, when 
encountering a stone or other obstacle on the ground, short 
pieces of smoked glass, an inch in height, were stuck upright 
into the sand in front of two thin lateral branches. Their tips 
scratched the smoked surface ia various directions; one made 
three upward and two downward lines, besides a nearly hori- 
zontal one; the other curled quite away from the glass; but 



Chap. IV. CIRCUMNUTATION OF STOLONS, 219 

ultimately both surmounted the glass and pursued their original 
course. The apex of a third thick stolon swept up the glass in 
a curved line, recoiled and again came into contact with it ; it 
then moved to the right, and after ascending, descended ver- 
tically ; ultimately it passed round one end of the glass instead 
of over it. 

Many long pins were next driven rather close together into 
the sand, so as to form a crowd in front of the same two thin 
lateral branches; but these easily wound their way through 
the crowd. A thick stolon was much delayed in its passage ; 
at one place it was forced to turn at right angles to its former 
course; at another place it could not pass through the pins, 
and the hinder part became bowed ; it then curved upwards 
and passed through an opening between the upper part of some 
pins w^hich happened to diverge ; it then descended and finally 
emerged through the crowd. This stolon was rendered perma- 
nently sinuous to a slight degree, and was thicker where sinu- 
ous than elsewhere, apparently from its longitudinal growth 
having been checked. 

Cotyledon umbilicus (Crassulaceae). — A plant growing in a 
pan of damp moss had emitted 3 stolons, 22 and 20 inches in 
length. One of these was supported, so that a length of 4^ 
inches projected in a straight and horizontal line, and the move- 
ment of the apex was traced. The first dot was made at 9. 10 
A.M. ; the terminal portion soon began to bend downwards and 
continued to do so until noon. Therefore a straight line, very 
nearly as long as the whole figure here given (Fig. 89), was first 
traced on the glass; but the upper part of this line has not been 
copied in the diagram. The curvature occurred in the middle 
of the penultimate internode ; and its chief seat was at the dis- 
tance of 1 J inch from the apex ; it appeared due to the weight 
of the terminal portion, acting on the more flexible part of the 
internode, and not to geotropism. The apex after thus sinking 
down from 9.10 a.m. to noon, moved a little to the left; it then 
rose up and circumnutated in a nearly vertical plane until 10.35 
P.M. On the following day (26th) it was observed from 6.40 
A.M. to 5.20 P.M., and within this time it moved twice up and 
twice down. On the morning of the 27th the apex stood as high 
as it did at 11.30 a.m. on the 25th. Nor did it sink down dup- 



220 



CIECUMNUTATION OF STOLONS. Chap. IV. 



ing the 28th, but continued to circumnutate about the same 
place. 

Another stolon, which resembled the last in almost every 
respect, was observed during the same two days, but only two 
inches of the terminal portion was allowed to project freely and 



^WlSa.m,2S^ ^'^-^^ 




e'iiO'mn^l^ 



Jfa.m.21^ 



Cotyledon umbilicus : circuraniitation of stolon, traced from 11.15 a.]\j:. 
Aug. 25th to 11 A.M. 27th. Plant ilhiininatecl from above. The 
terminal internode was "25 inch in length, the penultimate 2 '25, 
and the third 3"0 inches in length. Apex of stolon stood at a dis- 
tance of 575 inches from the vertical glass ; but it was not possi- 
ble to ascertain how much the tracing was magnified, as it was 
not known how great a length of the internode circumnutated. 



horizontally. On the 25th it continued from 9.10 a.m. to 1.30 
P.M. to bend straight downwards, apparently owing to its weight 
(Fig. 90); but after this hour until 10.35 p.m. it zigzagged. 
This fact deserves notice, for v/e here probably see the combined 



Chap. IV. CIRCUMNUTATION OF STOLONS. 



221 



effects of the bending down from weight and of circumnutation. 
The stolon, however, did not circumnutate when it first began 
to bend down, as may be observed in the present diagram, and 
as was still more evident in the last case, when a longer portion 
of the stolon was left unsupported. On the following day (2Gth) 
the stolon moved twice up and twice down, but still continued 
to fall; in the evening and during the night it travelled for 
some unknown cause in an oblique direction. 



lO^a^.J^.TTLS. 




Fig. 90. 



a°5'am. 



Gr4d'a.m.26^^ 



5"2Cl'^.m.2QH 



Wa-m i^ 0'40^a,m.27^ 

Cotyledon umbilicus : circumnutation and downward movement of an- 
other stolon, traced on vertical glass, from 9.11 a.m. Aug. 25th to 
11 A.M. 27th. Apex close to glass, so that figure but little magni- 
fied, and here reduced to two thirds of original size. 



We see from these three cases that stolons or runners 
circumnutate in a very complex manner. The lines 
generally extend in a vertical plane, and this may prob- 



222 CmCUMNUTATION OF FLOWER-STEMS. Chap. IV. 

ably be attributed to the effect of the weight of the un- 
supported end of the stolon ; but there is always some, 
and occasionally a considerable, amount of lateral move- 
ment. The circumnutation is so great in amplitude that 
it may almost be compared with that of climbing plants. 
That the stolons are thus aided in passing over obstacles 
and in winding between the stems of the surrounding 
plants, the observations above given render almost cer- 
tain. If they had not circumnutated, their tips would 
have been liable to have been doubled up, as often as 
they met with obstacles in their path ; but as it is, they 
easily avoid them. This must be a considerable advan- 
tage to the plant in spreading from its parent-stock ; but 
we are far from supposing that the power has been gained 
by the stolons for this purpose, for circumnutation seems 
to be of universal occurrence with all growing parts ; 
but it is not improbable that the amplitude of the move- 
ment may have been specially increased for this purpose. 

ClKCUMNUTATION OF FlOWER-STEMS. 

We did not think it necessary to make any special 
observations on the circumnutation of flower-stems, these 
being axial in their nature, like stems or stolons ; but 
some were incidentally made whilst attending to other 
subjects, and these we will here briefly give. A few 
observations have also been made by other botanists. 
These taken together suffice to render it probable that 
all peduncles and sub-peduncles circumnutate whilst 
growing. 

Oxalis camosa. — The peduncle which springs from the thick 
and woody stem of this plant bears three or four sub-peduncles. 
A filament with little triangles of paper was fixed within the 
calyx of a flower which stood upright. Its movements were 
observed for 48 h. ; during the first half of this time the flower 
was fully expanded, and during the second half withered. The 



Chap. IV. CIRC UMNUTATION OF FLOWER-STEMS. 223 

figure here given (Fig. 91) represents 8 or 9 ellipses. Although 
the main peduncle circumnutated, and described one large and 
two smaller ellipses in the course of 24 h., yet the chief seat of 
movement lies in the sub-peduncles, which ultimately bend ver- 
tically downwards, as will be described in a future chapter. 
The peduncles of Oxalis acetosella likewise bend downwards, and 
afterwards, when the pods are nearly mature, upwards ; and this 
is effected by a circum nutating movement. 

Fig. 91. 




Oxalis carnosa: flower-stem, feebly illuminated from above, its cir- 
cumnutation traced from 9 a.m. April 13th to 9 a.m. 15th. Sum- 
mit of flower 8 inches beneath the horizontal glass. Movement 
probably magnified about 6 times. 



It may be seen in the above figure that the flower-stem of 
0. carnosa circumnutated during two days about the same spot. 
On the other hand, the flower stem of 0. sefisitiva undergoes a 
strongly-marked, daily, periodical change of position, when kept 
at a proper temperature. In the middle of the day it stands 
vertically up, or at a high angle ; in the afternoon it sinks, and 
in the evening projects horizontally, or almost horizontally, ris- 
ing again during the night. This movement continues from the 
period when the flowers are in bud to when, as we believe, the 
pods are mature: and it ought perhaps to have been included 
amongst the so-called sleep-movements of plants. A tracino- 



224 CIRCUMNUTATION OF FLOWER-STEMS. Chap. IV. 

was not made, but the angles were measured at successive periods 
during one whole day; and these showed that the movement 
was not continuous, but that the peduncle oscillated up and 
down. We may therefore conclude that it circumnutated. At 
the base of the peduncle there is a mass of small cells, forming 
a well-developed pulvinus, which is exteriorly coloured purple 
and hairy. In no other genus, as far as we know, is the peduncle 
furnished with a pulvinus. The peduncle of 0. Ortegesii be- 
haved differently from that of 0. sensitiva, for it stood at a less 
angle above the horizon in the middle of the day, than in the 
morning or evening. By 10.20 p.m. it had risen greatly. 
During the middle of the day it oscillated much up and 
down. 

Trifolium subterraneum. — A filament was fixed vertically to 
the uppermost part of the peduncle of a young and upright 
flower-head (the stem of the plant having been secured to a 
stick) ; and its movements were traced during 36 h._ Within 
this time it described (see Fig. 92) a figure which represents four 
ellipses; but during the latter part of the time the peduncle 
began to bend downwards, and after 10.30 p.m. on the 24th it 
curved so rapidly down, that by 6.45 a.m. on the 25th it stood 
only 19° above the horizon. It went on circumnutating in nearly 
the same position for two days. Even after the flower-heads 
have buried themselves in the ground they continue, as will 
hereafter be shown, to circumnutate. It will also be seen in the 
next chapter that the sub-peduncles of the separate flowers of 
Trifolium repens circumnutate in a complicated course during 
several days, I may add that the gynophore of Arachis hypogcea, 
which looks exactly like a peduncle, circumnutates whilst grow- 
ing vertically downwards, in order to bury the young pod in 
the ground. 

The movements of the flowers of Cyclamen Fersicum were 
not observed ; but the peduncle, whilst the pod is forming, in- 
creases much in length, and bows itself down by a circumnu- 
tating movement. A young peduncle of Maurandia semper- 
florens, 1^ inch in length, was carefully observed during a 
whole day, and it made 4^ narrow, vertical, irregular and short 
ellipses, each at an average rate of about 2 h. 25 m. An ad- 
joining peduncle described during the same time similar, 



Chap. IV. CIRCUMNUTATION OF FLOWER-STEMS. 225 



though fewer, ellipses.* According to Sachs f the flower-stems, 
whilst growing, of many plants, for instance, those of Brassica 
napus, revolve or circumnutate ; those of Allium porrum bend 
from side to side, and, if this movement had been traced on a 



Fig. 92. 



6t40\a.m.?4¥ 



t40'a.m SSVfi 




y 



Trifolium subterraneum : main flower- peduncle, illuminated from 
above, circumnutation traced on horizontal glass, from 8.40 a.m. 
July 23rd to 10.30 p.m. 24th. 



horizontal glass, no doubt ellipses would have been formed. 
Fritz Muller has described I the spontaneous revolving move- 
ments of the flower-stems of an Alisma, which he compares 
with those of a climbing plant. 



* The Movements and Habits of many plants occupy different 



of Climbing Plants,' 2nd edit. 
1875, p. 68. 

t 'Text-Book of Botany,' 1875, 
p. 766. Linnaeus and Treviranus 
(according to Pfeffer, ' Die Pe- 
riodischen Bewegungen,' &c. p. 
162) state that the flower-stalks 



positions by night and day, and 
we shall see in the chapter on 
the Sleep of Plants that this im- 
plies circumnutation. 

X ' Jenaische Zeitsch.,' B. v. p. 
133. 



226 CIRCUMNUTATION OF LEAVES. Chap. IV. 

We made no observations on the movements of the different 
parts of flowers. Morren, h >wever, has observed * in the sta- 
mens of Sparmannia and Cereus a ' ' fremissement spontane," 
which, it may be suspected, is a circumnutating movement. 
The circumnutation of the gynostemium of Stylidium, as de- 
scribed by Gad,t is highly remarkable, and apparently aids in 
the fertilisation of the flowers. The gynostemium, whilst spon- 
taneously moving, comes into contact with the viscid labellum, 
to which it adheres, until freed by the increasing tension of the 
parts or by being touched. 

We have now seen that the flower-stems of plants be- 
longing to such widely different families as the Crucif- 
erae, Oxalidae, Leguminosse, Primulaceae, Scrophularinese, 
Alismacese, and Liliaceae, circumnutate ; and that there 
are indications of this movement in many other families. 
With these facts before us, bearing also in mind that the 
tendrils of not a few plants consist of modified pedun- 
cles, we may admit without much doubt that all grow- 
ing flower-stems circumnutate. 



'O 



Circumnutation of Leaves : Dicotyledons. 

Several distinguished botanists, Hofmeister, Sachs, 
Pfeffer, De Vries, Batalin, Millardet, &c., have observed, 
and some of them with the greatest care, the periodical 
movements of leaves ; but their attention has been chiefly, 
though not exclusively, directed to those which move 
largely and are commonly said to sleep at night. From 
considerations hereafter to be given, plants of this nature 
are here excluded, and will be treated of separately. As 
we wished to ascertain whether all young and growing 
leaves circumnutated, we thought that it would be suffi- 
cient if we observed between 30 and 40 genera widely dis- 



"'•■ ' N. Mem. de rAcad. R. de t ' Sitzmisrsboricht des bot. Ver- 
Bruxelles,' torn. xiv. 1841, p. 3. eins der P. Brandenburg.' xxi. 

p. 84. 



Chap. IV. 



DICOTYLEDONS. 



22; 



tributed throughout the vegetable series, selecting some 
unusual forms and others on woody plants. All the plants 
were healthy and grew in pots. They were illuminated 
from above, but the light perhaps was not always suffi- 
ciently bright, as many of them were observed under a 
skylight of ground-glass. Except in a few specified 
cases, a fine glass filament with two minute triangles of 
paper was fixed to the leaves, and their movements were 
traced on a vertical glass (when not stated to the con- 
trary) in the manner already described. I may repeat 
that the broken lines represent the nocturnal course. 
The stem was always secured 
to a stick, close to the base of 
the leaf under observation. 
The arrangement of the species, 
Avith the number of the Family 
appended, is the same as in the 
case of stems. 



Fig. 93. 




Sarracenia purpurea : circum- 
nutation of young pitcher, 
traced from 8 a.m. July 3rd 
to 10.15 A.M. 4th. Temp. 
17°-18° C. Apex of pitcher 
20 inches from glass, so 
movement greatly magni- 
fied. 



(1.) Sarracenia purpurea (Sar- 
raceneae, Fam. 11), — A young leaf, 
or pitcher, 8| inches in height, 
with the bladder swollen, but with 
the hood not as yet open, had a 
filament fixed transversely across 
its apex ; it was observed for 48 h., 
and during the whole of this time 

it circumnutated in a nearly similar manner, but to a very small 
extent. The tracing given (Fig. 93) relates only to the move- 
ments during the first 26 h. 

(2.) Olaucium luteum (Papaveraceae, Fam. 12). — A young 
plant, bearing only 8 leaves, had a filament attached to the 
youngest leaf but one, which was 3 inches in length, including 
the petiole. The circumnutating movement was traced during 
47 h. On both days the leaf descended from before 7 a.m. un- 
til about 11 A.M., and then ascended slightly during the rest of 
the day and the early part of the night. During the latter part 



228 



CmOUMNUTATION OF LEAVES. Chap. IV. 



of the night it fell greatly. It did not ascend so much during 
the second as during the first day, and it descended consider- 
ably lower on the second night 
than on the first. This difference 
was probably due to the illumina- 
tion from above having been in- 
suflScient during the two days of 
observation. Its course during the 
two days is shown in Fig. 94. 

(3.) Crambe maritima (Crucif- 
erse, Fam. 14). — A leaf 9| inches 
in length on a plant not growing 
vigorously was first observed. Its 
apex was in constant movement, 
but this could hardly be traced, 
from being so small in extent. 
The apex, however, certainly 
changed its course at least 6 
times in the course of 14 h. A 
more vigorous young plant, bear- 
ing only 4 leaves, was then select- 
ed, and a filament was affixed to 
the midrib of the third leaf from 
the base, which, with the petiole, 
■ was 5 inches in length. The leaf 

\ stood up almost vertically, but the 

• tip was deflected, so that the fila- 

/: ment projected almost horizon- 

tally, and its movements were 
traced during 48 h. on a vertical 
glass, as shown in the accom- 
panying figure (Fig. 95). We 
here plainly see that the leaf was 
continually circumnutating; but 
the proper periodicity of its move- 
ments was disturbed by its being 
only dimly illuminated from above through a double skylight. 
We infer that was the case, because two leaves on plants grow- 
ing out of doors, had their angles above the horizon measured 




Glaucium luteiim : circumnuta- 
tion of young leaf, traced 
from 9.30 a.m. June 14th to 
8.30 A.M. 16th. Tracing not 
much magnified, as apex of 
leaf stood only 5i inches 
from the glass. 



Chap. IV. 



DICOTYLEDONS. 



'2-29 



Fi?. 95. 



6°^'a.m.25^ 



7:5(fa.m.2S^ 



in the middle of the day and at 9 to about 10 p.m. on succes- 
sive nights, and they were found at this latter hour to have 
risen by an average 
angle of 9° above their 
midday position: on 
the following morn- 
ing they fell to their 
former position. Now 
it may be observed in 
the diagram that the 
leaf rose during the 
second night, so that 
it stood at 6.40 a.m. 
higher than at 10.20 
P.M. on the preceding 
night; and this may 
be attributed to the 
leaf adjusting itself to 
the dim light, com- 
ing exclusively from 
above. 

(4.) Brassica olera- 
eea (Cruciferae). — Hof- 
meister and Batalin * 
state that the leaves 
of tbe cabbage rise at 
night, and fall by day. 
We covered a young 
plant, bearing 8 leaves, 
under a large bell- 
glass, placing it in 
the same position with 
respect to the light in 
which it had long remained, and a filament was fixed at the 
distance of -4 of an inch from the apex of a young leaf neariy 
4 inches in length. Its movements were then traced during 
three days, but the tracing is not wori:h giving. The leaf fell 




6^S0i'cun24f^ 



3'£.m^4^ 



Cramhe mariiima : circumnutation of leaf, 
disturbed by being insnfficiently illumi- 
nated from above, traced from 7.50 a.m. 
June 23rd to S a.m. 25th. Apex of leaf 
15i inches from the vertical glass, so that 
the tracing was much magnified, but is 
here reduced to one- fourth of original scale. 



Flora," 1873. p. 43: 



230 CmCUMNUTATION OF LEAVES. Crap. IV. 

during the whole morning, and rose in the evening and during 
the early part of the night. The ascending and descending 
lines did not coincide, so that an irregular ellipse was formed 
each 34 h. The basal part of the midrib did not move, as was 
ascertained by measuring at successive periods the angle which 
it formed with the horizon, so that the movement was confined 
to the terminal portion of the leaf, which moved through an 
angle of 11° in the course of 24 h., and the distance travelled by 
the apex, up and down, was between -8 and "9 of an inch. 

In order to ascertain the effect of darkness, a filament was 
fixed to a leaf 5^ inches in length, borne by a plant which after 
forming a head had produced a stem. The leaf was inclined 
44° above the horizon, and its movements were traced on a ver- 
tical glass every hour by the aid of a taper. During the first 
day the leaf rose from 8 a.m. to 10.40 p.m. in a slightly zigzag 
course, the actual distance travelled by the apex being "67 of an 
inch. During the night the leaf fell, whereas it ought to have 
risen; and by 7 a.m. on the following morning it had fallen -23 
of an inch, and it continued falling until 9.40 a.m. It then 
rose until 10.50 p.m., but the rise was interrupted by one con- 
siderable oscillation, that is, by a fall and re-ascent. During 
the second night it again fell, but only to a very short distance, 
and on the following morning re-ascended to a very short dis- 
tance. Thus the normal course of the leaf was greatly disturbed, 
or rather completely inverted, by the absence of light, and the 
movements were likewise greatly diminished in amplitude. 

We may add that, according to Mr. A. Stephen Wilson,* 
the young leaves of the Swedish turnip, which is a hybrid be- 
tween B. oleracea and rapa, draw together in the evening so 
much "that the horizontal breadth diminishes about 30 per 
cent, of the daylight breadth." Therefore the leaves must rise 
considerably at night. 

(5.) Dianthus caryopliyllus (Caryophylleae, Fam. 26). — The 
terminal shoot of a young plant, growing very vigorously, was 
selected for observation. The young leaves at first stand up 
vertically and close together, but they soon bend outwards and 



*■ 'Trans. Bot. Soc. Edinburgh.' see Darwin, 'Animals and Plants 
vol. xiii. p. 32. With respect to under Domestication,' 2nd edit., 
the origin of the Swedish turnip, vol. i. p. 344. 



Chap. IV. 



DICOTYLEDONS. 



231 



downwards, so as to become horizontal, and often at the same 
time a little to one side. A filament was fixed to the tip of a 
young leaf whilst still highly inclined, and the first dot was 
made on the vertical glass at 8.30 a.m. June 13th, but it curved 
downwards so quickly that by 6.40 a.m. on the following morn- 
ing it stood only a little above the horizon. In Fig. 96 the long, 



Fig. 96. 



fiCiJ/j 




lD°3ffp.mJ£^ 




'15'p.mJ3^ 



I \e''4o'a,m.24$ 






Dianthus caryophyllus : circumnutation of young leaf, traced from 10.15 
P.M. June 13th to 10.35 p.m. 16th. Apex of leaf stood, at the close 
of our observations, 8| inches from the vertical glass, so tracing 
not greatly magnified. The leaf was 5| inches long. Temp. 15^°- 

• i7r c. 



slightly zigzag line representing this rapid downward course, 
which was somewhat inclined to the left, is not given ; but the 
figure shows the highly tortuous and zigzag course, together 
with some loops, pursued during the next 2^ days. As the leaf 
continued to move all the time to the left, it is evident that the 
zigzag line represents many circumnutations. 

(6.) Camellia Japonica (Camelliaceae, Fam. 32). — A youngish 
leaf, which together with its petiole was 2f inches in length and 
which arose from a side branch on a tall bush, had a filament 
attached to its apex. This leaf sloped downwards at an angle 
of 40° beneath the horizon. As it was thick and rigid, and its 
16 



232 



CIRCUMNUTATIOX OF LEAVES. Chap. IV. 




petiole very short, much movement could not be expected. 

Nevertheless, the apex changed its course completely seven 
times in the course of 11^ h., but 
moved to only a very small distance. 
On the next day the movement of 
the apex was traced during 26 h. 
20 m. (as shown in Fig. 97), and 
was nearly of the same nature, but 
rather less complex. The move- 
ment seems to be periodical, for on 
both days the leaf circumnutated in 
the forenoon, fell in the afternoon 
(on the first day until between 3 

and 4 p.m. and on the second day until 6 p.m.), and then rose, 

falling again during the night or early morning. 

\9'. 30' amM^h 



Camellia Japonica : circumnu- 
tation of leaf, traced from 
6.40 A.M. June 14th to 6.50 
A.M. 15th. Apex of leaf 12 
inches from the vertical 
glass, so figure considera- 
bly magnified. Temp. 16°- 
16^ C. 




flO''.45\p.m» 






e^.so'p.m.ieth . 

Pelargonium zonale: circumnutation and downward movement of 
young leaf, traced from 9.30 A.M. June 14th to 6.30 p.m. 16th. 
Apex of leaf 91 inches from the vertical glass, so figure moderately 
magnified. Temp. 15°-16.r C. 

In the chapter on the Sleep of Plants we shall see that the 
leaves in several Malvaceous genera sink at night; and as they 



Chap. IV. 



DICOTYLEDONS. 



233 



Fig. 99. 



often do not then occupy a vertical position, especially if they 
have not been well illuminated during the day, it is doubtful 
whether some of these cases ought not to have been included in 
the present chapter. 

(7.) Pelargonium zonale (Geraniaceae, Fam. 47), — A young 
leaf, IJ inch in breadth, with its petiole 1 inch long, borne on 
a young plant, was observed in the usual manner during 61 h. ; 
and its course is shown in the preceding figure (Fig. 98). 
During the first day and night the leaf moved downwards, but 
circumnutated between 10 a.m. and 4.30 p.m. On the second 
day it sank and rose again, but be- 
tween 10 A.M. and 6 p.m. it circum- 
nutated on an extremely small scale. 
On the third day the circumnutation 
was more plainly marked. 

(8.) Cissus discolor (Ampelidese, 
Fam. 67). — A leaf, not nearly full- 
grown, the third from the apex of a 
shoot on a cut-down plant, was ob- 
served during 31 h. 30 m. (see Fig. 
99). The day. was cold (15°-16° C), 
and if the plant had been observed in 
the hot - house, the circumnutation, 
though plain enough as it was, would 
probably have been far more con- 
spicuous. 

(9.) Vicia fdba (Leguminosse, 
Fam. 75). — A young leaf, 3'1 inches 
in length, measured from base of pe- 
tiole to end of leaflets, had a filament 
affixed to the midrib of one of the two terminal leaflets, and its 
movements were traced during 51^ h. The filament fell all 
morning (July 2nd) till 3 p.m., and then rose greatly till 10.35 
P.M. ; but the rise this day was so great, compared with that 
which subsequently occurred, that it was probably due in part 
to the plant being illuminated from above. The latter part of 
the course on July 2nd is alone given in the following figure 
(Fig. 100). On the next day (July 3rd) the leaf again fell in 
the morning, then circumnutated in a conspicuous manner, 




Cissus discolor: circumnu- 
tation of leaf, traced 
from 10.35 A.M. May 28th 
to 6 P.M. 29th. Apex of 
leaf 8| inches from the 
vertical glass. 



234 



CIRCUMNUTATION OF LEAVES. Chap. IV. 



and rose till late at night ; but the movement was not traced 
after 7.15 p.m., as by that time the filament pointed towards 
the upper edge of the glass. During the latter part of the night 
or early morning it again fell in the same manner as before. 



Fig. 100. 



6°^^a.in.3n^l 




'6°4S'a.m.4^ 



°J5'a.m 4^ 



Viciafaha: circuinnntation of leaf, traced from 7.15 P.M. July 2nd to 
10.15 A.M. 4th. Apex of the two terminal leaflets 7i inches from 
the vertical glass. Figure here reduced to two-thirds of original 
scale. Temp. 17°-18° C. 



As the evening rise and the early morning fall were un- 
usually large, the angle of the petiole above the horizon was 



Chap. IV. 



DICOTYLEDONS. 



235 



measured at the two periods, and the leaf was found to have 
risen 19° between 12.20 p.m. and 10.45 p.m., and to have fallen 
33° 30' between the latter hour and 10.20 a.m. on the following 
morning. 

The main petiole was now secured to a stick close to the 
base of the two terminal leaflets, which were 1 '4 inch in leng-th ; 
and the movements of one of them were traced during 48 h. (see 
Fig. 101). The course pursued is closely analogous to that of 



Fig. 101. 



JiO°J5'jp.m/a^. 




JO'30'a.m.e^ 



JO'40'a.m. 4'> 



Vicia faba : circumnutation of one of the two terminal leaflets, the 
main petiole having been secured, traced from 10.40 a.m. July 4th 
to 10.30 a.m. 6th. Apex of leaflet 61 inches from the vertical glass. 
Tracing here reduced to one-half of original scale. Temp. 16°- 
18° C. •= i' ^ 



the whole leaf. The zigzag line between 8.30 a.m. and 3.30 
P.M. on the second day represents 5 very small ellipses, with 



236 



CIRCUMNUTATION OF LEAVES. Chap. IV. 




Acacia retinoides : cir- 
cumnutation of a 
young phyllode, 
traced from 10.45 
A.M. July 18th to 
8 . 15 A. M. 19th. 
Apex of phyllode 9 
inches from the ver- 
tical glass. Temp. 
16r-17i° C. 



their longer axes differently directed. From these observations 
it follows that both the whole leaf and the terminal leaflets 
undergo a well-marked daily periodical 
movement, rising in the evening and fall- 
ing during the latter part of the night or 
early morning; whilst in the middle of the 
day they generally 
circumnutate round 
the same small space. 
(10.) Acacia reti- 
noides (Leguminosse). 
— The movement of 
a young phyllode, 2f 
inches in length, and 
inclined at a consid- 
erable angle above 
the horizon, was 
traced during 45 h. 
30 m. ; but in the 
figure here given 
(Fig. 102), its circumnutation is shown 
during only 21 h. 80 m. During part of 
this time (viz., 14 h. 30 m.) the phjilode 
described a figure representing 5 or 6 
small ellipses. The actual amount of 
movement in a vertical direction was '3 
inch. The phyllode rose considerably be- 
tween 1.30 P.M. and 4 p.m., but there was 
no evidence on either day of a regular 
periodic movement. 

(11.) Lupinus speciosus (Leguminosse). 
— Plants were raised from seed purchased 
under this name. This is one of the spe- 
cies in this large genus, the leaves of 
which do not sleep at night. The pe- 
tioles rise direct from the ground, and are 
from 5 to 7 inches in length. A filament 
was fixed to the midrib of one of the longer leaflets, and the 
movement of the whole leaf was traced, as show^n in Fiff. 103. 




Lnpimis speciosus : cir- 
cumnutation of leaf 
traced on vertical 
glass, from 10.15 a.m. 
to 5.45 P.M.; i.e., dur- 
ing 6 h. 30 m. 



Chap. IV. 



DICOTYLEDONS. 



23Y 



Fiff. 104 



In the course of 6 h. 30 m. the filament went four times up and 
three times down. A new tracing was then begun (not here 
given), and during 12^ h. the leaf moved eight times up and 
seven times down ; so that it described 7^ ellipses in this time, 
and this is an extraordinary rate of movement. The summit 
of the petiole was then secured to a stick, and the separate leaf- 
lets were found to be continually circumnutating. 

(12.) Echeveria stolonifera (Crassulacese, Fam. 84). — The 
older leaves of this plant are so thick and fleshy, and the young 
ones so short and broad, that it 
seemed very improbable that any 
circumnutation could be detected. 
A filament was fixed to a young 
upwardly inclined leaf, -75 inch in 
length and -28 in breadth, which 
stood on the outside of a terminal 
rosette of leaves, produced by a 
plant growing very vigorously. 
Its movement was traced during 3 
days, as here shown (Fig. 104). 
The course was chiefly in an up- 
ward direction, and this may be 
attributed to the elongation of the 
leaf through growth; but we see 
that the lines are strongly zigzag, 
and that occasionally there was 
distinct circumnutation, though 
on a very small scale. 

(13.) Bryophyllum (vel Galan- 
cTim) calycinum (Crassulacese). — 
Duval-Jouve ('Bull. Soc. Bot. de France,' Feb. 14, 1868) 
measured the distance between the tips of the upper pair of 
leaves on this plant, with the result shown in the following 
Table. It should be noted that the measurements on Dec. 2nd 
were made on a different pair of leaves : — 




Echeveria stolonifera: circum- 
nutation of leaf, traced from 
8.20 A.M. June 25th to 8.45 
A.M. 28th. Apex of leaf 121 
inches from the glass, so 
that the movement was 
much magnified. Temp. 
23°-24r C. 





8 A.M. 


2 p.m. 


7 p.m. 


Nov. 16 . 


. 15 mm. . 


. . 25 mm. . 


. . (?) 


" 19 . 


. . 48 " 


. . 60 " 


. 48 mm 


Dec. 2 . 


. . 22 " 


. . 43 " 


. . 28 " 



238 



CIRCUMNUTATION OF LEAVES. Chap. IV. 



We see from this Table that the leaves stood considerably fur- 
ther apart at 2 p.m. than at either 8 a.m. or 7 p.m. ; and this 
shows that they rise a little in the evening and fall or open in 
the forenoon. 

(14.) Drosera ratundifolia (Droserace^, Fam. 85). — The 
movements of a young leaf, having a long petiole but with its 

tentacles (or gland-bearing 



m45'pml^} 



Fig. 105. _^ hairs) as yet unfolded, were 

traced during 47 h. 15 m. 
The figure (Fig. 105) shows 
that it circumnutated large- 
ly, chiefly in a vertical di- 
rection, making two ellipses 
each day. On both days the 
leaf began to descend after 
13 or 1 o'clock, and con- 
tinued to do so all night, 
though to a very unequal 
distance on the two occa- 
sions. We therefore thought 
that the movement was pe- 
riodic; but on observing 
three other leaves during 
several successive days and 
nights, we found this to be 
an error; and the case is 
given merely as a caution. 
On the third morning the 
above leaf occupied almost 
exactly the same position as 
on the first morning; and 
the tentacles by this time 
had unfolded sufficiently to 
project at right angles to 
the blade or disc. 
The leaves as they grow older generally sink more and more 
downwards. The movement of an oldish leaf, the glands of 
which were still secreting freely, was traced for 24 h., during 
which time it cont-inued to sink a little in a slightly zigzag line. 




i0°40'p.m\8fl^ 



9'15a:m:iK 



Drosera rotundifolia : circumnutation 
of yotmg leaf, with filament fixed 
to back of blade, traced from 9.15 
A.M. June 7tli to 8.30 a.m. June 
9th. Figure hero reduced to one- 
half original scale. 



Chap. IV. DICOTYLEDONS. 239 

On the following morning, at 7 a.m., a drop of a solution of 
carbonate of ammonia (2 gr. to 1 oz. of water) was placed on 
the disc, and this blackened the glands and induced inflection 
of many of the tentacles. The weight of the drop caused the 
leaf at first to sink a little ; but immediately afterwards it be- 
gan to rise in a somewhat zigzag course, and continued to do 
so till 3 P.M. It then circumnutated about the same spot on a 
very small scale for 21 h. ; and during the next 21 h. it sank in 
a zigzag line to nearly the same level which it had held when 
the ammonia was first administered. By this time the tentacles 
had re-expanded, and the glands had recovered their proper 
colour. We thus learn that an old leaf circumnutates on a small 
scale, at least whilst absorbing carbonate of ammonia ; for it is 
probable that this absorption may stimulate growth and thus 
re-excite circumnutation. Whether the rising of the glass fila- 
ment which was attached to the back of the leaf, resulted from 
its margin becoming slightly inflected (as generally occurs), or 
from the rising of the petiole, was not ascertained. 

In order to learn whether the tentacles or gland-bearing 
hairs circumnutate, the back of a young leaf, with the inner- 
most tentacles as yet incurved, was firmly cemented with shel- 
lac to a flat stick driven into compact damp argillaceous sand. 
The plant was placed under a microscope with the stage re- 
moved and with an eye-piece micrometer, of which each division 
equalled -^^q of an inch. It should be stated that as the leaves 
grow older the tentacles of the exterior rows bend outwards and 
downwards, so as ultimately to become deflected considerably 
beneath the horizon. A tentacle in the second row from the 
margin was selected for observation, and was found to be mov- 
ing outwards at a rate of -^^^ of an inch in 20 m., or ^^ of inch 
in 1 h. 40 m. ; but as it likewise moved from side to side to an 
extent of above ^fg of inch, the movement was probably one of 
modified circumnutation. A tentacle on an old leaf was next 
observed in the same manner. In 15 m. after being placed 
under the microscope it had moved about y-g^^^ of an inch. 
During the next 7i h. it was looked at repeatedly, and during 
this whole time it moved only another y-J^^ of an inch ; and this 
small movement may have been due to the settling of the damp 
sand (on which the plant rested), though the sand had been 



240 



CmCUMNUTATION OF LEAVES. Chap. IV. 



Fig. 106. 



firmly pressed down. We may therefore conclude that the ten- 
tacles when old do not ch'cumnutate ; yet this tentacle was so 
sensitive, that in 23 seconds after its gland had been merely 
touched with a bit of raw meat, it began to curl inwards. This 
fact is of some importance, as it appar- 
ently shows that the inflection of the 
tentacles from the stimulus of absorbed 
animal matter (and no doubt from that 
of contact with any object) is not due to 
modified circumnutation. 

(15.) Dioncea miiscipula (Droseracese). 
— It should be premised that the leaves 
at an early stage of their development 
have the two lobes pressed closely to- 
gether. These are at first directed back 
towards the centre of the plant ; but they 
gradually rise up and soon stand at right 
angles to the petiole, and ultimately in 
nearly a straight line with it. A young 
leaf, which with the petiole was only 1-3 
inch in length, had a filament fixed ex- 
ternally along the midrib of the still 
closed lobes, w^hich projected at right 
angles to the petiole. In the evening 
this leaf completed an ellipse in the 
course of 2 h. On the following day 
(Sept. 25th) its movements were traced 
during 22 h. ; and we see in Fig. 106 
that it moved in the same general direc- 
tion, due to the straightening of the leaf, 
but in an extremely zigzag line. This 
line represents several drawn - out or 
modified ellipses. There can therefore 
be no doubt that this young leaf circumnutated. 

A rather old, horizontally extended leaf, with a filament 
attached along the under side of the midrib, was next observed 
during 7 h. It hardly moved, but when one of its sensitive 
hairs was touched, the blades closed, though not very quickly. 
A new dot was now made on the glass, but in the course of 




Dionxa mtiscipula : cir- 
cumnutation of a 
young and expanding 
leaf, traced on a hori- 
zontal glass in dark- 
ness, from noon Sept. 
24th to 10 A.M. 25th. 
Apex of leaf 13^ 
inches from the glass, 
so tracing consider- 
ably magnified. 




Chap. IV. DICOTYLEDONS. 241 

14 h. 20 m. there was hardly any change in the position of the 
filament. We may therefore infer that an old and only moder- 
ately sensitive leaf does not circumnutate plainly; but we 
shall soon see that it by no means follows that such a leaf is 
absolutely motionless. We may further infer that the stimulus 
from a touch does not re-excite plain circumnutation. 

Another full-grown leaf had a filament attached externally 
along one side of the midrib and parallel to it, so that the fila- 
ment would move if the lobes closed. It should be first stated 
that, although a touch on one of the sensitive hairs of a vigor- 
ous leaf causes it to close quickly, often almost instantly, yet 
when a bit of damp meat or some solution of carbonate of am- 
monia is placed on the lobes, they close so slowly that generally 
24 h. is required for the completion of the act. The above leaf 

Fig. 107. 

8V5>^. jo'46'pm.25i 
^. , ^ ^ ^ _ , . ^ 'Ba.m.26^1' 

Dionsea musciptda : closure of the lobes and circumnutation of a full- 
grown leaf, whilst absorbing an infusion of raw meat, traced in 
darkness, from 7.15 p.m. Sept. 24th to 9 a.m. 26th. Apex of leaf 
8^ inches from the vertical glass. Figure here reduced to two- 
thirds of original scale. 

was first observed for 2 h. 30 m., and did not circumnutate, but 
it ought to have been observed for a longer period ; although, 
as we have seen, a young leaf completed a fairly large ellipse in 
2 h. A drop of an infusion of raw meat was then placed on the 
leaf, and within 2 h. the glass filament rose a little ; and this 
implies that the lobes had begun to close, and perhaps the peti- 
ole to rise. It continued to rise with extreme slowness for the 
next 8 h. 30 m. The position of the pot was then (7.15 p.m., 
Sept. 24th) slightly changed and an additional drop of the infu- 
sion given, and a new tracing was begun (Fig. 107). By 10.50 
P.M. the filament had risen only a little more, and it fell during 
the night. On the following morning the lobes were closing 
more quickly, and by 5 p.m. it was evident to the eye that they 
had closed considerably; by 8.48 p.m. this was still plainer, and 
by 10.45 P.M. the marginal spikes were interlocked. The leaf 
fell a little during the night, and next morning (25th) at 7 a.m. 



242 CIRCUMNUTATION OF LEAVES. Chap. IV. 

the lobes were completely shut. The course pursued, as may 
be seen in the figure, was strongly zigzag, and this indicates 
that the closing of the lobes was combined with the circumnu- 
tation of the whole leaf, and there cannot be much doubt, con- 
sidering how motionless the leaf was during 2 h. 30 m. before 
it received the infusion, that the absorption of the animal mat- 
ter had excited it to circumnutate. The leaf was occasionally 
observed for the next four days, but was kept in rather too cool 
a place ; nevertheless it continued to circumnutate to a small 
extent, and the lobes remained closed. 

It is sometimes stated in botanical works that the lobes clo^e 
or sleep at night ; but this is an error. To test the statement, 
very long glass filaments were fixed inside the two lobes of 
three leaves, and the distances between their tips were measured 
in the middle of the day and at night ; but no difference could 
be detected. 

The previous observations relate to the movements of the 
whole leaf, but the lobes move independently of the petiole, and 
seem to be continually opening and shutting to a very small 
extent. A nearly full-grown leaf (afterwards proved to be 
highly sensitive to contact) stood almost horizontally, so that 
by driving a long thin pin through the foliaceous petiole close 
to the blade, it was rendered motionless. The plant, with a 
little triangle of paper attached to one of the marginal spikes, 
was placed under a microscope with an eye-piece micrometer, 
each division of which equalled j^ of an inch. The apex of 
the paper-triangle was now seen to be in constant slight move- 
ment; for in 4 h. it crossed nine divisions, or j^-^ of an inch, 
and after ten additional hours it moved back and had crossed 
■jf^ ill ^° opposite direction. The plant was kept in rather too 
cool a place, and on the following day it moved rather less, 
namely, -^^ in 3 h. , and ^f „ in an opposite direction during the 
next 6 h. The two lobes, therefore, seem to be constantly clos- 
ing or opening, though to a very small distance ; for we must 
remember that the little triangle of paper affixed to the marginal 
spike increased its length, and thus exaggerated somewhat the 
movement. Similar observations, with the important difference 
that the petiole was left free and the plant kept under a high 
temperature, were made on a leaf, which was healthy, but so 



Chap- IV. DICOTYLEDONS. 243 

old that it did not close when its sensitive hairs were repeat- 
edly touched, though judging from other cases it would have 
slowly closed if it had been stimulated by animal matter. The 
apex of the triaogie was in almost, though not quite, constant 
movement, sometimes in one direction and sometimes in an 
opposite one ; and it thrice crossed five divisions of the microm- 
eter (i.e. Ywo ^f ^^ inch) in 30 m. This movement on so small 
a scale is hardly comparable with ordinary circumnutation ; but 
it may perhaps be compared with the zigzag lines and little 
loops, by which the larger ellipses made by other plants are 
often interrupted. 

In the first chapter of this volume, the remarkable oscillatory 
movements of the circumnutating hypocotyl of the cabbage 
have been described. The leaves of Dionaea present the same 
phenomenon, which is a wonderful one, as viewed under a low 
power (2-inch object-glass), with an eye-piece micrometer of 
which each division (yfg- of an inch) appeared as a rather wide 
space. The young unexpanded leaf, of which the circumnu- 
tating movements were traced (Fig. 106), had a glass filament 
fixed perpendicularly to it ; and the movement of the apex was 
observed in the hot-house (temp. 84° to 86° F.), with light 
admitted only from above, and with any lateral currents of air 
excluded. The apex sometimes crossed one or two divisions of 
the micrometer at an imperceptibly slow rate, but generally it 
moved onwards by rapid starts or jerks of ^^ or y-g^o^ and in 
one instance of ^ oVo of an inch. After each jerk forwards, the 
apex drew itself backwards with comparative slowness for part 
of the distance which had just been gained ; and then after a 
very short time made another jerk forwards. Four conspicuous 
jerks forwards, with slower retreats, were seen on one occasion 
to occur in exactly one minute, besides some minor oscillations. 
As far as we could judge, the advancing and retreating lines 
did not coincide, and if so, extremely minute ellipses were each 
time described. Sometimes the apex remained quite motion- 
less for a short period. Its general course during the several 
hours of observation was in two opposite directions, so that the 
leaf was probably circumnutating. 

An older leaf with the lobes fully expanded, and which was 
afterwards proved to be highly sensitive to contact, was next 



244 CmCUMNUTATION OF LEAVES. Chap. lY. 

observed in a similar manner, except that the plant was exposed 
to a lower temperature in a room. The apex oscillated forwards 
and backwards in the same manner as before ; but the jerks for- 
ward were less in extent, viz. about -j-^qq inch ; and there were 
longer motionless periods. As it appeared possible that the 
movements might be due to currents of air, a wax taper was 
held close to the leaf during one of the motionless periods, but 
no oscillations were thus caused. After 10 m., however, vigor- 
ous oscillations commenced, perhaps owing to the plant having 
been warmed and thus stimulated. The candle was then re- 
moved and before long the oscillations ceased; nevertheless, 
when looked at again after an interval of 1 h. 30 m., it was 
again oscillating. The plant was taken back into the hot-house, 
and on the following morning was seen to be oscillating, though 
not very vigorously. Another old but healthy leaf, which was 
not in the least sensitive to a touch, was likewise observed dur- 
ing two days in the hot-house, and the attached filament made 
many little jerks forwards of about Y^-g- or only y^jVo of an inch. 

Finally, to ascertain whether the lobes independently of th& 
petiole oscillated, the petiole of an old leaf was cemented close 
to the blade with shellac to the top of a little stick driven into 
the soil. But before this was done the leaf was observed, and 
found to be vigorously oscillating or jerking ; and after it had 
been cemented to the stick, the oscillations of about jq\^ of 
an inch still continued. On the following day a little infusion 
of raw meat was placed on the leaf, which caused the lobes to 
close together very slowly in the course of two days ; and the 
oscillations continued during this whole time and for the next 
two days. After nine additional days the leaf began to open 
and the margins were a little everted, and now the apex of the 
glass filament remained for long periods motionless, and then 
moved backwards and forwards for a distance of about y-Jg-^ of 
an inch slowly, without any jerks. Nevertheless, after warm- 
ing the leaf with a taper held close to it, the jerking movement 
recommenced. 

This same leaf had been observed 2^ months previously, and 
was then found to be oscillating or jerking. We may therefore 
infer that this kind of movement goes on night and day for a 
very long period ; and it is common to young unexpanded leaves 



Chap. IV. 



DICOTYLEDONS. 



245 



and to leaves so old as to have lost their sensitiveness to a touch, 
but which were still capable of absorbing nitrogenous matter. 
The phenomenon when well displayed, as in the young leaf just 
described, is a very interesting one. It often brought before 
our minds the idea of effort, or of a small animal struggling to 
escape from some constraint. 

(16.) Eucalyptus resinifera (Myrtacese, Fam. 94). — A young 
leaf, two inches in length together with the petiole, produced 
by a lateral shoot from a cut-down tree, was observed in the 
usual manner. The blade had not as yet assumed its vertical 
position. On June 7th only a few observations were made, and 
tlie tracing merely showed that the leaf had moved three times 
upwards and three downwards. 
On the following day it was 
observed more frequently; and 
two tracings were made (see A 
and B, Fig. 108), as a single 
one would have been too com- 
plicated. The apex changed 
its course 13 times in the course 
of 16 h., chiefly up and down, 
but with some lateral move- 
ment. The actual amount of 
movement in any one direction 
was small. 

(17.) Dahlia (garden var.) 
(Composites, Fam. 122). — A fine 
young leaf 5| inches in length, 
produced by a young plant 2 feet high, growing vigorously in a 
large pot, was directed at an angle of about 45° beneath the 
horizon. On June 18th the leaf descended from 10 a.m. till 
11.35 A.M. (see Fig. 109); it then ascended greatly till 6 p.m., 
this ascent being probably due to the light coming only from 
above. It zigzagged between 6 p.m. and 10.35 p.m., and 
ascended a little during the night. It should be remarked 
that the vertical distances in the lower part of the diagram are 
much exaggerated, as the leaf was at first deflected beneath 
the horizon, and after it had sunk downwards, the filament 
pointed in a very oblique line towards the glass. Next day 




Eucalyptus resinifera : circumnu- 
tation of a leaf, traced, A, from 
6.40 A.M. to 1 P.M. June 8tli ; 
B, from 1 p.m. 8th to 8.30 a.m. 
9th. Apex of leaf 14^ inches 
from the horizontal glass, so 
figures considerably magnified. 



246 



CIRCUMNUTATION OF LEAVES. Chap. IV. 



the leaf descended from 8.20 a.m. till 7.15 p.m., then zig- 
zagged and ascended greatly during the night. On the morn- 
ing of the 20th the leaf was probably beginning to descend, 
though the short line in the diagram is horizontal. The actual 
distances travelled by the apex of the leaf were considerable, 



Fig. 109. 




10^ 



Dahlia: circumnutation of leaf, traced from 10 A.M. June 18th. to 8.10 
A.M. 20th, but with a break of 1 b. 40 m. on the morning of the 
19th, as, owing to the glass filament pointing too much to one side, 
the pot had to be slightly moved ; therefore the relative position 
of the two tracings is somewhat arbitrary. The figure here given 
is reduced to one-fifth of the original scale. Apex of leaf 9 inches 
from the glass in the line of its inclination, and 4| in a horizontal 
line. 



but could not be calculated with safety. From the course pur- 
sued on the second day, when the plant had accommodated 
itself to the light from above, there cannot be much doubt that 
the leaves undergo a daily periodic movement, sinking during 
the day and rising at night. 



Chap. IV. DICOTYLEDONS. 247 

(18.) Mutisia clematis (Compositae). — The leaves terminate 
in tendrils and circumnutate like those of other tendril-bearers ; 
but this plant is here mentioned, on account of an erroneous 
statement * which has been published, namely, that the leaves 
sink at night and rise during the day. The leaves v^^hich 
behaved in this manner had been kept for some days in a 
northern room and had not been sufficiently illuminated. A 
plant therefore was left undisturbed in the hot-house, and three 
leaves had their angles measured at noon and at 10 p.m. All 
three were inclined a little beneath the horizon at noon, but one 
stood at night 2°, the second 21°, and the third 10° higher than 
in the middle of the day ; so that instead of sinking they rise 
a little at night. 

(19.) Cyclamen Persicum (Primulacese, Fam. 135). — A young 
leaf, 1 "8 of an inch in length, petiole included, produced by an 
old root-stock, was observed during three days in the usual 
manner (Fig. 110). On the first day the leaf fell more than 
afterwards, apparently from adjusting itself to the light from 
above. On all three days it fell from the early morning to 
about 7 P.M., and from that hour rose during the night, the 
course being slightly zigzag. The movement therefore is strict- 
ly periodic. It should be noted that the leaf would have sunk 
each evening a little lower down than it did, had not the glass 
filament rested between 5 and 6 p.m. on the rim of the pot. 
The amount of movement was considerable ; for if we assume 
that the whole leaf to the base of the petiole became bent, the 
tracing would be magnified rather less than five times, and this 
would give to the apex a rise and fall of half an inch, with 
some lateral movement. This amount, however, would not at- 
tract attention without the aid of a tracing or measurement of 
some kind. 

(20.) Allamanda Schottii (Apocynese, Fam. 144). — The young 
leaves of this shrub are elongated, with the blade bowed so much 
downwards as almost to form a semicircle. The chord — that is, 
a line drawn from the apex of the blade to the base of the peti- 
ole — of a young leaf, 4| inches in length, stood at 2.50 p.m. on 
Dec. 5th at an angle of 13° beneath the horizon, but by 9.30 p.m. 



*' ' The Movements and Habits of Climbing Plants,' 1875, p. 118. 
17 



24:8 



CIRCtTMNUTATION OF LEAVES. Chap. IV. 



tlie blade had straightened itself so much, which implies the 
raising of the apex, that the chord now stood at 37° above 
the horizon, and had therefore risen 50°. On the next day 

Fig. 110. 



S'SQ*a.m.S^ 




^6'40'a.m.S^ 



Cyclamen Persic7im: circumnutation of leaf, traced from 6.45 a.m. June 
STnd to 6.40 A.M. 5th. Apex of leaf 7 inches from the vertical glass. 



similar angular measurements of the same leaf were made ; and 
at noon the chord stood 36° beneath the horizon, and 9.30 p.m. 



Chap. IV. 



DICOTYLEDONS. 



249 



Fig. 111. 



3^° above it, so had risen 39^°. The chief cause of the rising 

movement lies in the straightening of the blade, but the short 

petiole rises between 4° and 5°. On the third night the chord 

stood at 35° above the horizon, 9 

and if the leaf occupied the 

same position at noon, as on 

the previous day, it had risen 

71°. With older leaves no such 

change of curvature could be 

detected. The plant was then 

brought into the house and 

kept in a north-east room, but 



in the curvature of the young 
leaves; so that previous expo- 
sure to a strong light is appar- 
ently requisite for the periodi- 
cal change of curvature in the 
blade, and for the slight rising 
of the petiole. 

(21.) Wigandia (Hydrolea- 
cese, Fam. 149). — Professor 
Pfeffer informs us that the 
leaves of this plant rise in the 
evening; but as we do not 
know w^hether or not the rising 
is great, this species ought per- 
haps to be classed amongst 
sleeping plants. 

(22.) Petunia molacea (Sola- 
nese, Fam. 157). — Avery young 
leaf, only f inch in length, 
highly inclined upwards, was 
observed for four days. Dur- 
ing the whole of this time it 
bent outwards and downw^ards, 
so as to become more and 
more nearly horizontal. The 
strongly marked zigzag line in 




Petunia molacea : downward move- 
ment and circumnutation of a 
very young leaf, traced from 10 
A.M. June 2nd to 9.20 A.M. June 
6th. N.B.— At 6.40 a.m. on the 
5th it was necessary to move the 
pot a little, and a new tracing 
was begun at the point where 
two dots are not joined in the 
diagram. Apex of leaf 7 inches 
from the vertical glass. Temp, 
generally 17^° C. 



250 



CIRCUMNUTATION OF LEAVES. Chap. IV. 



the figure on p. 349 (Fig. Ill), shows that this was effected by 
modified circumnutation ; and during the latter part of the time 
there was much ordinary circumnutation on a small scale. The 
movement in the diagram is magnified between 10 and 11 times. 
It exhibits a clear trace of periodicity, as the leaf rose a little 



Fig. 112 



conquered by the leaf striving to become more and more hori- 
zontal as it grew older. The angles which two older leaves 
formed together, were measured in 
the evening and about noon on 3 
successive days, and each night the 
angle decreased a little, though ir- 
regularly. 

(23.) Acanthus mollis (Acantha- 
ceae, Fam. 168). — The younger of two 
leaves, 2^ inches in length, petiole 
included, produced by a seedling 
plant, was observed during 47 h. 
Early on each of the three morn- 
ings, the apex of the leaf fell; and 
it continued to fall till 3 p.m., on 
the two afternoons when observed. 
After 3 p.m. it rose considerably, 
and continued to rise on the second 
night until the early morning. But 
on the first night it fell instead of 
rising, and we have little doubt 
that this was owing to the leaf 
being very young and becoming 
through epinastic growth more and 
more horizontal ; for it may be seen 
in the diagram (Fig. 112), that the 
leaf stood on a higher level on the 
first than on the second day. The 
leaves of an allied species {A. spi- 
nosus) certainly rose every night ; and the rise between noon and 
10.15 P.M., when measured on one occasion, was 10°. This rise 
was chiefly or exclusively due to the straightening of the blade, 
and not to the movement of the petiole. We may therefore 




Acanthus mollis : circumnuta- 
tion of young leaf, traced 
from 9.20 a.m. June 14th 
to 8.30 A.M. 16th. Apex 
of leaf 11 inches from the 
vertical glass, so move- 
ment considerably magni- 
fied. Figure here i educed 
to one-half of original scale. 
Temp. 15°-16r C. 



Chap. IV. DICOTYLEDONS. 251 

conclude that the leaves of Acanthus circumnutate periodically, 
falling in the morning and rising in the afternoon and night. 

(24.) Cannabis sativa (Cannabiness, Fam. 195). — We have 
here the rare case of leaves moving downwards in the evening, 
but not to a sufficient degree to be called sleep.* In the early 
morning, or in the latter part of the night, they move upwards. 
For instance, all the young leaves near the summits of several 
stems stood almost horizontally at 8 a.m. May 29th, and at 
10.30 P.M. were considerably declined. On a subsequent day two 
leaves stood at 2 p.m. at 21° and 12° beneath the horizon, and at 
10 P.M. at 38° beneath it. Two other leaves on a younger plant 
were horizontal at 2 p.m., and at 10 p.m. had sunk to 36° beneath 
the horizon. With respect to this downward movement of the 
leaves, Kraus believes that it is due to their epinastic growth. 
He adds, that the leaves are relaxed during the day, and tense 
at night, both in sunny and rainy weather. 

(25.) Pinus pinaster (ConifersB, Fam. 223). — The leaves on 
the summits of the terminal shoots stand at first in a bundle 
almost upright, but they soon diverge and ultimately become 
almost horizontal. The movements of a young leaf, nearly one 
inch in length, on the summit of a seedling plant only 3 inches 
high, w ere traced from the early morning of June 2nd to the 
evening of the 7th. During these five days the leaf diverged, 
and its apex descended at first in an almost straight line ; but 
during the two latter days it zigzagged so much that it was 
evidently circumnutating. The same little plant, when grown 
to a height of 5 inches, was again observed during four days. 
A filament was fixed transversely to the apex of a leaf, one inch 
in length, and which had already diverged considerably from 
its originally upright position. It continued to diverge (see A, 
Fig. 113), and to descend from 11.45 a.m. July 31st to 6.40 a.m. 
Aug. 1st. On August 1st it circumnutated about the same 
small space, and again descended at night. Next morning the 
pot was moved nearly one inch to the right, and a new tracing 
was begun (B). From this time, viz., 7 a.m. August 2nd to 



* We were led to observe this Flora, 1879, p. 66. We regret that 

plant by Dr. Carl Kraus' paper, we cannot fully understand parts 

' Beitrage zur Kentniss der Bewe- of this paper, 
gungen Wachsender Laubbliitter,' 



252 



CmCUMNUTATION OF LEAVES. Chap. IV. 



8.20 A.M. on the 4th, the leaf manifestly circumnutated. It 
does not appear from the diagram that the leaves move periodi- 
cally, for the descending course during the first two nights, 



Fig. 113. 



I A. 



e%0'am.nt 




6''40'a.m 




8'20'a.m^ 



-7°«.m.5"^ 



Finns pinaster : circiimnutation of young leaf, traced from 11 45 A M 
July 31st to 8.20 a.m. Aug. 4th. At 7 a.m. Aug. 2nd the pot was 
moved an inch to one side, so that the tracing consists of two fig- 
ures. Apex of leaf 14i inches from the vertical glass, so move- 
ments much magnified. 

was clearly due to epinastic growth, and at the close of our 
observations the leaf was not nearly so horizontal as it would 
ultimately become. 



J 



Chap. IV. 



MONOCOTYLEDONS. 



253 



114. 



Pinus austriaca. — Two leaves, 3 inches in length, but not 
quite fully grown, produced by a lateral shoot, on a young tree 
3 feet in height, were observed during 29 h. (July 31st), in the 
same manner as the leaves of the pre- 
vious species. Both these leaves cer- 
tainly circumnutated, making within 
the above period two, or two and a 
half, small, irregular ellipses. 

(26.) Cycas pectinata (Cycade^, 
Fam. 224). — A young leaf, 11|^ inches 
in length, of which the leaflets had 
only recently become uncurled was 
observed during 47 h. 30 m. The 
main petiole was secured to a stick at 
the base of the two terminal leaflets. 
To one of the latter, 3| inches in 
length, a filament was fixed ; the leaf- 
let was much bowed downward, but 
as the terminal part was upturned, the 
filament projected almost horizontally. 
The leaflet moved (see Fig. 114) large- 
ly and periodically, for it fell until 7 
P.M. and rose during the night, fall- 
ing again next morning after 6.40 a.m. 
The descending lines are in a marked 
manner zigzag, and so probably would 
have been the ascending lines, if they 
had been traced throughout the night. 



clrcumi^utation of leaves 
Monocotyledons. 



Cycas pectinata : circum- 
nutation of one of the 
terminal leaflets, traced 
from 8.30 A.M. June 
22nd to 8 A.M. June 
24th. Apex of leaflet 
7i inches from the ver- 
tical glass, so tracing 
not greatly magnified, 
and here reduced to 
one - third of original 
scale. Temp. 19°-21° C. 



in breadth, produced by 



(27.) Ganna Warscewicsii (Canna- 
ce86, Fam. 2). — The movements of a 
young leaf, 8 inches in length and 3^ 
a vigorous young plant, were observed during 45 h. 50 m., as 
shown in Fig. 115. The pot was slided about an inch to the 
right on the morning of the 11th, as a single figure would have 
been too complicated; but the two figures are continuous in 



254 



CIRCUMNUTATION OF LEAVES. Chap. IV. 



time. The movement is periodical, as the leaf descended from 
the early morning until about 5 p.m., and ascended during the 
rest of the evening and part of the night. On the evening of 




A B 

Canna Warscewiczii : circumnntation of leaf, traced f A) from 11.30 a.m. 
June loth to 6.40 A.M. 11th ; and (B) from 6.40 A.M 11th to 8.40 
A.M. 12th. Apex of leaf 9 inches from the vertical glass. 



Fig. 116. 



the 11th it circumnutated on a small scale for some time about 
the same spot. 

(28.) Iris pseudo - acorns (Iridese, Fam. 10). — The move- 
ments of a young leaf, rising 13 inches above the water in 
which the plant grew, were traced 
as shown in the figure (Fig. 116), 
during 27 h. 30 m. It manifestly 
circumnutated, though only to a 
small extent. On the second morn- 
ing, between 6.40 a.m. and 2 p.m. 
(at which latter hour the figure here 
given ends), the apex changed its 
course five times. During the next 
8 h. 40 m. it zigzagged much, and 
descended as far as the lowest dot 
in the figure, making in its course 
two very small ellipses ; but if these 
lines had been added to the dia- 




Iris pseudo-neorus : circum- 
nutation of leaf, traced 
from 10.30 A.M. May 28th 
to 2 P.M. 29th. Tracing 
continued toll p.m., but 
not here copied. Apex 
of leaf 12 inches beneath 
the horiz(mtal glass, so 
figuve considerably mag- 
nified. Temp. 15°-16°C. 



gram it would have been too com- 



plex. 



Chap. IV. MONOCOTYLEDONS. 255 

(29.) Crinum Capense (Amaryllidese, Fam. 11). — The leaves 
of this plant are remarkable for their great length and narrow- 
ness; one was measured and found to be 53 inches long and 
only 1*4 broad at the base. Whilst quite young they stand up 
almost vertically to the height of about a foot ; afterwards their 
tips begin to bend over, and subsequently hang vertically down, 
and thus continue to grow. A rather young leaf was selected, 
of which the dependent tapering point was as yet only 5^ inches 
in length, the upright basal part being 20 inches high, though 
this part would ultimately become shorter by being more bent 
over. A large bell-glass was placed over the plant, with a 
black dot on one side ; and by bringing the dependent apex of 

Fig. 117. 



Crinum capense : circumnutation of dependent tip of young leaf, traced 
on a bell-glass, from 10.30 p.m. May 22nd to 10.15 a.m. 25th. Fig- 
ure not greatly magnified. 

the leaf into a line with this dot, the accompanying figure (Fig, 
117) was traced on the other side of the bell, during 2^ days. 
During the first day (26nd) the tip travelled laterally far to the 
left, perhaps in consequence of the plant having been disturbed ; 
and the last dot made at 10.30 p.m. on this day is alone here 
given. As we see in the figure, there can be no doubt that the 
apex of this leaf circumnutated. 

A glass filament with little triangles of paper was at the 
same time fixed obliquely across the tip of a still younger leaf, 
which stood vertically up and was as yet straight. Its move- 
ments were traced from 3 p.m. May 22nd to 10.15 a.m. 25th. 
The leaf was growing rapidly, so that the apex ascended greatly 
during this period ; as it zigzagged much it was clearly circum- 



256 CmCUMNUTATION OF LEAVES. Chap. IV. 

nutating, and it apparently tended to form one ellipse each day. 
The lines traced during the night were much more vertical 
than those traced during the day; and this indicates that the 
tracing would have exhibited a nocturnal rise and a diurnal 
fall, if the leaf had not grown so quickly. The movement of 
this same leaf after an interval of six days (May 3 1st), by which 
time the tip had curved outwards into a horizontal position, 
and had thus made the first step towards becoming dependent, 
was traced orthogonically by the aid of a cube of wood (in the 
manner before explained) ; and it was thus ascertained that the 
actual distance travelled by the apex, and due to circumnuta- 
tion, was 3^ inches in the course of 20| h. During the next 
24 h. it travelled 2} inches. The circumnutating movement, 
therefore, of this young leaf was strongly marked. 

(30.) Pancratium littorale (Amaryllidese). — The movements, 
much magnified, of a leaf, 9 inches in length and inclined at 
about 45° above the horizon, were traced during two days. On 
the first day it changed its course completely, upwards and 
downwards and laterally, 9 times in 12 h. ; and the figure 
traced apparently represented five ellipses. On the second 
day it was observed seldomer, and was therefore not seen to 
change its course so often, viz., only 6 times, but in the same 
complex manner as before. The movements were small in 
extent, but there could be no doubt about the circumnutation 
of the leaf. 

(31.) ImatopTiyllum ml Glima (sp.?) (Amaryllidese). — Along 
glass filament was fixed to a leaf, and the angle formed by it 
with the horizon was measured occasionally during three suc- 
cessive days. It fell each morning until between 3 and 4 p.m., 
and rose at night. The smallest angle at any time above the 
horizon was 48°, and the largest 50° ; so that it rose only 2° 
at night; but as this was observed each day, and as similar 
observations were nightly made on another leaf on a distinct 
plant, there can be no doubt that the leaves move period- 
ically, though to a very small extent. The position of the 
apex when it stood highest was -8 of an inch above its lowest 
point. 

(32.) Pistia stratiofes (Aroidens, Fam. 30).— Hofmeister re- 
marks that the leaves of this floating water-plant are more 



Chap. IV. MONOCOTYLEDONS. 257 

highly inclined at night than by day.* We therefore fastened 
a fine glass filament to the midrib of a moderately young leaf, 
and on Sept. 19th measured the angle which it formed with the 
horizon 14 times between 9 a.m. and 11.50 p.m. The tempera- 
ture of the hot-house varied during the two days of observation 
between 18^ and 23|° C. At 9 a.m. the filament stood at 32° 
above the horizon; at 3.34 p.m. at 10° and at 11.50 p.m. at 55°; 
these two latter angles being the highest and the lowest observed 
during the day, showing a difference of 45°. The rising did 
not become strongly marked until between 5 and 6 p.m. On 
the next day the leaf stood at only 10° above the horizon at 
8.25 a.m., and it remained at about 15° till past 3 p.m; at 5.40 
p.m. it was 23°, and at 9.30 p.m. 58° ; so that the rise was more 
sudden this evening than on the previous one, and the differ- 
ence in the angle amounted to 48°. The movement is obviously 
periodical, and as the leaf stood on the first night at 55°, and 
on the second night at 58° above the horizon, it appeared very 
steeply inclined. This case, as we sliall see in a future chapter, 
ought perhaps to have been included under the head of sleep- 
ing plants. 

Fig. 118. 




Pontederia fsp. ?) : circumuutation of leaf, traced from 4.50 p.m. July 
• 2ncl to 10.15 a.m. 4th. Apex of leaf 16i inches from the vertical 
glass, so tracing greatly magnified. Temp, about 17° C, and there- 
fore rather too low. 

(33.) Pontederia (sp. ?) (from the highlands of St. Catharina, 
Brazil) (Pontederiaceae, Fam. 46).— A filament was fixed across 



* 'Die Lehre von der Pflanzenzellc,' 1867, p. 427. 



258 CmCUMNUTATION OF CRYPTOGAMS. Chap. IV. 

the apex of a moderately young leaf, 7|- inches in height, and 
its movements were traced during 42i h. (see Fig. 118). On 
the first evening, when the tracing was begun, and during the 
night, the leaf descended considerably. On the next morning 
it ascended in a strongly marked zigzag line, and descended 
again in the evening and during the night. The movement, 
therefore, seems to be periodic, but some doubt is thrown on 
this conclusion, because another leaf, 8 inches in height, appear- 
ing older and standing more highly inclined, behaved differ- 
ently. During the first 12 h. it circumnutated over a small 
space, but during the night and the whole following day it 
ascended in the same general direction ; the ascent being effected 
by repeated up and down well-pronounced oscillations. 

Cryptogams. 

(34.) NepTirodium molle (Filices, Fam. 1). — A filament was 
fixed near the apex of a young frond of this Fern, 17 inches 
in height, which was not as yet fully uncurled ; and its move- 
ments were traced during 34 h. We see in Fig. 119 that it 
plainly circumnutated. The movement w^as not greatly mag- 
nified as the frond was placed near to the vertical glass, and 




Fig. 119. 



Nephrodium molle : circumnutation of rachis, traced from 9.15 A.M. May 
28th to 9 A.M. 29th. Figure here given two-thirds of original scale. 



would probably have been greater and more rapid had the day 
been warmer. For the plant was brought out of a warm green- 
house and observed under a skylight, where the temperature 



Chap. IV. CIRCUMNUTATION OE^ CRYPTOGAMS. 



259 



Fio;. 120. 



was between 15° and 16° C. We have seen in Chap. I. that a 
frond of this Fern, as yet only slightly lobed and with a rachis 
only '23 inch in height, plainly 
circumnutated. * 

In the chapter on the Sleep 
of Plants the conspicuous cir- 
cumnutation of Marsilea quadri- 
foUata (Marsileacese, Fam. 4) 
will be described. 

It has also been shown in 
Chap. I. that a very young Sela- 
ginella (Lycopodiaceae, Fam. 6), 
only "4 inch in height, plainly 
circumnutated; we may there- 
fore conclude that older plants, 
whilst growing, would do the 
same. 

(35.) Lunularia 'vulgaris (He- 
patic^, Fam. 11, Muscales). — 
The earth in an old flower-pot 
was coated with this plant, bear- 
ing gemmae, A highly inclined 
frond, which projected -3 inch 
above the soil and was '4 inch in 
breadth, was selected for obser- 
vation. A glass hair of extreme 
tenuity, '75 inch in length, with 
its end whitened, was cemented 
with shellac to the frond at 
right angles to its breadth ; and a white stick with a minute 
black spot was driven into the soil close behind the end of the 




Lunularia vulgaris •' circumnuta- 
tioxi of a frond, traced from 
9 A.M. Oct. 25tli to 8 A.M. 27th. 



* Mr. Loomis and Prof. Asa 
Gray have described ( ' Botanical 
Gazette,' 1880, pp. 27, 43), an ex- 
tremely curious case of move- 
ment in the fronds, but only in 
the fruitiug frouds, of Asplenium 
trichomanes. They move almost 
as rapidly as tbe little leaflets 
of Desmodium (lyrans, alternately 
backwards and forwards tbrougli 
from 20 to 40 degrees, in a plane at 



right angles to that of the frond. 
The apex of the frond describes ' ' a 
long and veiy narrow ellipse," so 
that it circumnutates. But the 
movement differs from ordinary 
circumnutation as it occurs only 
when the plant is exposed to the 
light; even artificial light "is 
suflicient to excite motion for a 
few minutes." 



260 CIRCUMNUTATION OF CRYPTOGAMS. Chap. IV. 

hair. The white end could be accurately brought into a line 
with the black spot, and dots could thus be successively made 
on the vertical glass-plate in front. Any movement of the 
frond would of course be exhibited and increased by the long 
glass hair ; and the black spot was placed so close behind the 
end of the hair, relatively to the distance of the glass-plate in 
front, that the movement of the end was magnified about 40 
times. Nevertheless, we are convinced that our tracing gives a 
fairly faithful representation of the movements of the frond. 
In the intervals between each observation, the plant was covered 
by a small bell-glass. The frond, as already stated, was highly 
inclined, and the pot stood in front of a north-east window. 
During the five first days the frond moved downwards or became 
less inclined; and the long line which was traced was strongly 
zigzag, with loops occasionally formed or nearly formed ; and 
this indicated circumnutation. Whether the sinking was due 
to epinastic growth, or apheliotropism, we do not know. As 
the sinking was slight on the fifth day, a new tracing was begun 
on the sixth day (Oct. 25th), and was continued for 47 h. ; it is 
here given (Fig. 120), Another tracing was made on the next 
day (27th) and the frond was found to be still circumnutating, 
for during 14 h. 30 m. it changed its course completely (besides 
minor changes) 10 times. It was casually observed for two 
more days, and was seen to be continually moving. 

The lowest members of the vegetable series, the Thallogens, 
apparently circumnutate. If an Oscillaria be watched under 
the microscope, it may be seen to describe circles about every 
40 seconds. After it has bent to one side, the tip first begins 
to bend back to the opposite side and then the whole filament 
curves over in the same direction. Hofmeister* has given a 
minute account of the curious, but less regular though constant, 
movements of Spirogyra: during 2|^ h. the filament moved 4 
times to the left and 3 times to the right, and he refers to a 
movement at right angles to the above. The tip moved at the 
rate of about O'l mm. in five minutes. He compares the move- 



* ' Uehor die BowegunsPn dor liindisclic Naturkunde in Wiirt- 
Fiiden dor Spirogyra princeps : temberg,' 1874, p. 211. 
Jalireshcfte dcs Vercins fiir vatcr- 



Chap. IV. CmCUMNUTATION OF LEAVES. 261 

ment with the nutation of the higher plants.* We shall here- 
after see that heliotropic movements result from modified cir- 
cumnutation, and as unicellular Moulds bend to the light we 
may infer that they also circumnutate. 

Concluding Eemarks on the Cikcumnutation 
OE Leaves. 

The circumnutating movements of young leaves in 
33 genera, belonging to 25 families, widely distributed 
amongst ordinary and gymnospermous Dicotyledons and 
amongst Monocotyledons, together with several Crypto- 
gams, have now been described. It would, therefore, 
not be rash to assume that the growing leaves of all 
plants circumnutate, as we have seen reason to conclude 
is the case with cotyledons. The seat of movement 
generally lies in the petiole, but sometimes both in the 
petiole and blade, or in the blade alone. The extent of 
the movement differed much in different plants ; but the 
distance passed over was never great, except with Pistia, 
which ought perhaps to have been included amongst 
sleeping plants. The angular movement of the leaves 
was only occasionally measured ; it commonly varied 
from only 2° (and probably even less in some instances) 
to about 10° ; but it amounted to 23° in the common 
bean. The movement is chiefly in a vertical plane, but 
as the ascending and descending lines never coincided, 
there was always some lateral movement, and thus ir- 
regular ellipses were formed. The movement, therefore, 
deserves to be called one of circumnutation ; for all cir- 
cumnutating organs tend to describe ellipses,— that is, 
growth on one side is succeeded by growth on nearly but 



* Zukal also remarks fas quoted her of the Oscillatorieoe, are closely 

in 'Journal R. Microscop. Soc.,' analogous "to the well-known 

1880, vol. iii. p. 320) that the rotation of groAving shoots and 

movements of Spirulina, a mem- tendrils." 



262 CIRCUMNUTATION OF LEAVES. Ghap. IV. 

not quite the opposite side. The ellipses, or the zig- 
zag lines representing drawn-out ellipses, are generally 
very narrow; yet with the Camellia, their minor axes 
were half as long, and with the Eucalyptus more than 
half as long as their major axes. In the case of Cissus, 
parts of the figure more nearly represented circles than 
ellipses. The amount of lateral movement is therefore 
sometimes considerable. Moreover, the longer axes of 
the successively formed ellipses (as with the Bean, Cissus, 
and Sea-kale), and in several instances the zigzag lines 
representing ellipses, were extended in very different di- 
rections during the same day or on the next day. The 
course followed was curvilinear or straight, or slightly 
or strongly zigzag, and little loops or triangles were often 
formed. A single large irregular ellipse may be described 
on one day, and two smaller ones by the same plant on 
the next day. With Drosera two, and with Lupinus, 
Eucalyptus and Pancratium, several were formed each 
day. 

The oscillatory and jerking movements of the leaves 
of Dionaea, which resemble those of the hypocotyl of the 
cabbage, are highly remarkable, as seen under the micro- 
scope. They continue night and day for some months, 
and are displayed by young unexpanded leaves, and by 
old ones which have lost their sensibility to a touch, but 
which, after absorbing animal matter, close their lobes. 
We shall hereafter meet with the same kind of move- 
ment in the joints of certain (rramineae, and it is prob- 
ably common to many plants while circumnutating. 
It is, therefore, a strange fact that no such movement 
could be detected in the tentacles of Drosera rotundi- 
folia^ though a member of the same family with Dionaea ; 
yet the tentacle which was observed was so sensitive, that 
it began to curl inwards in 23 seconds after being touched 
by a bit of raw meat. 



Chap. IV. CIRCUMNUTATION OF LEAVES. 263 

One of the most interesting facts with respect to the 
circumnutation of leaves is the periodicity of their 
movements; for they often, or even generally, rise a 
little in the evening and early part of the night, and 
sink again on the following morning. Exactly the 
same phenomenon was observed in the case of cotyle- 
dons. The leaves in 16 genera out of the 33 which 
were observed behaved in this manner, as did probably 

2 others. Nor must it be supposed that in the remain- 
ing 15 genera there was no periodicity in their move- 
ments ; for 6 of them were observed during too short a 
period for any judgment to be formed on this head, and 

3 were so young that their epinastic growth which serves 
to bring them down into a horizontal position, over- 
powered every other kind of movement. In only one 
genus, Cannabis, did the leaves sink in the evening, and 
Kraus attributes this movement to the prepotency of 
their epinastic growth. That the periodicity is deter- 
mined by the daily alternations of light and darkness 
there can hardly be a doubt, as will hereafter be shown. 
Insectivorous plants are very little affected, as far as 
their movements are concerned, by light ; and hence 
probably it is that their leaves, at least in the cases of 
Sarracenia, Drosera, and Dionaea, do not move periodi- 
cally. The upward movement in the evening is at first 
slow, and with different plants begins at very differ- 
ent hours; — with Glaucium as early as 11 a.m., com- 
monly between 3 and 5 p.m., but sometimes as late as 
7 P.M. It should be observed that none of the leaves 
described in this chapter (except, as we believe, those of 
Lupi?ius speciosus) possess a pulvinus ; for the periodi- 
cal movements of leaves thus provided have generally 
been amplified into so - called sleep - movements, with 
which we are not here concerned. The fact of leaves 
and cotvledons frequently, or even generally, rising a 

18 



264: CmCUMNUTATION OF LEAVES. Chap. IV. 

little in the evening and sinking in the morning, is of 
interest as giving the foundation from which the special- 
ised sleep-movements of many leaves and cotyledons, not 
provided with a pulvinus, have been developed. The 
above periodicity should be kept in mind, by any one 
considering the problem of the horizontal position of 
leaves and cotyledons during the day, whilst illuminated 
from above. 



CHAPTEE V. 
Modified Circumnutation : Climbing Plants : Epinastic 

AND HyPONASTIC MOVEMENTS. 

Circumnutation modified through innate causes or through the action 
of external conditions — Innate causes — Climbing plants ; similarity 
of their movements with those of ordinary plants ; increased am- 
plitude ; occasional points of difierence — Epinastic growth of 
young leaves — Hyponastic growth of the hypocotyls and epicotyls 
of seedlings — Hooked tips of climbing and other plants due 
to modified circumnutation — Ampelopsis tricuspidata — Smithia 
Pfundii — Straightening of the tip due to hyponasty — Epinastic 
growth and circumnutation of the flower-peduncles of Trifolium 
repens and Oxalis carnosa. 

The radicles, h3^pocotyls and epicotyls of seedling 
plants, even before they emerge from the ground, and 
afterwards the cotyledons, are all continually circum- 
nutating. So it is with the stems, stolons, flower -pe- 
duncles, and leaves of older plants. We may, therefore, 
infer with a considerable degree of safety that all the 
growing parts of all plants circumnutate. Although 
this movement, in its ordinary or unmodified state, ap- 
pears in some cases to be of service to plants, either 
directly or indirectly — for instance, the circumnutation 
of the radicle in penetrating the ground, or that of the 
arched hypocotyl and epicotyl in breaking through the 
surface — yet circumnutation is so general, or rather so 
universal a phenomenon, that we cannot suppose it to 
have been gained for any special purpose. We must 
believe that it follows in some unknown way from the 
manner in which vegetable tissues grow. 

265 



266 MODIFIED CIRCUMNUTATION. Chap. V. 

We shall now consider the many cases in which 
circumnutation has been modified for various special 
purposes; that is, a movement already in progress is 
temporarily increased in some one direction, and tempo- 
rarily diminished or quite arrested in other directions. 
These cases may be divided in two sub-classes ; in one 
of which the modification depends on innate or consti- 
tutional causes, and is independent of external condi- 
tions, excepting in so far that the proper ones for 
growth must be present. In the second sub-class the 
modification depends to a large extent on external 
agencies, such as the daily alternations of light and 
darkness, or light alone, temperature, or the attraction 
of gravity. The first small sub-class will be considered 
in the present chapter, and the second sub-class in the 
remainder of this volume. 

The Ciecumnutation of Olimbii^g Plakts. 

The simplest case of modified circumnutation is that 
offered by climbing plants, with the exception of those 
which climb by the aid of motionless hooks or of root- 
lets : for the modification consists chiefly in the greatly 
increased amplitude of the movement. This would fol- 
low either from greatly increased growth over a small 
length, or more probably from moderately increased 
growth spread over a considerable length of the mov- 
ing organ, preceded by turgescence, and acting succes- 
sively on all sides. The circumnutation of climbers 
is more regular than that of ordinary plants; but in 
almost every other respect there is a close similarity 
between their movements, namely, in their tendency 
to describe ellipses directed successively to all points 
of the compass — in their courses being often inter- 
rupted by zigzag lines, triangles, loops, or small ellipses 
— in the rate of movement, and in different species 



Chap. V. CLIMBING PLANTS. 267 

revolving once or several times within the same length 
of time. In the same internode, the movements cease 
first in the lower part and then slowly upwards. In 
both sets of cases the movement may be modified in a 
closely analogous manner by geotropism and by heli- 
otropism ; though few climbing plants are heliotropic. 
Other points of similarity might be pointed out. 

That the movements of climbing plants consist of 
ordinary circumnutation, modified by being increased 
in amplitude, is well exhibited whilst the plants are very 
young ; for at this early age they move like other seed- 
lings, but as they grow older their movements gradually 
increase without undergoing any other change. That 
this power is innate, and is not excited by any ex- 
ternal agencies, beyond those necessary for growth and 
and vigour, is obvious. No one doubts that this power 
has been gained for the sake of enabling climbing plants 
to ascend to a height, and thus to reach the light. This 
is effected by two very different methods ; first, by twin- 
ing spirally round a support, but to do so their stems 
must be long and flexible ; and, secondly, in the case of 
leaf-climbers and tendril-bearers, by bringing these or- 
gans into contact with a support, which is then seized 
by the aid of their sensitiveness. It may be here re- 
marked that these latter movements have no relation, as 
far as we can judge, with circumnutation. In other 
cases the tips of tendrils, after having been brought into 
contact with a support, become developed into little discs 
which adhere firmly to it. 

We have said that the circumnutation of climbing 
plants differs from that of ordinary plants chiefly by 
its greater amplitude. But most leaves circumnutate in 
an almost vertical plane, and therefore describe very 
narrow ellipses, whereas the many kinds of tendrils 
which consist of metamorphosed leaves, make much 



268 MODIFIED CIRCUMNUTATION. Chap. V. 

broader ellipses or nearly circular figures ; and thus they 
have a far better chance of catching hold of a support 
on any side. The movements of climbing plants have 
also been modified in some few other special ways. 
Thus the circumnutating stems of Solnanum dulcamara 
can twine round a support only when this is as thin 
and flexible as a string or thread. The twining stems 
of several British plants cannot twine round a support 
when it is more than a few inches in thickness ; whilst 
in tropical forests some can embrace thick trunks ; * 
and this great difference in power depends on some 
unknown difference in their manner of circum nuta- 
tion. The most remarkable special modification of this 
movement which we have observed is in the tendrils 
of Echinocystis lohata ; these are usually inclined at 
about 45° above the horizon, but they stiffen and 
straighten themselves so as to stand upright in a part 
of their circular course, namely, when they approach 
and have to pass over the summit of the shoot from 
which they arise. If they had not possessed and exer- 
cised this curious power, they would infallibly have 
struck against the summit of the shoot and been ar- 
rested in their course. As soon as one of these tendrils 
with its three branches begins to stiffen itself and rise 
up vertically, the revolving motion becomes more rapid ; 
and as soon as it has passed over the point of difficulty, 
its motion coinciding with that from its own weight, 
causes it to fall into its previously inclined position so 
quickly, that the apex can be seen travelling like the 
hand of a gigantic clock. 

A large number of ordinary leaves and leaflets and 
a few flower-peduncles are provided with pulvini ; but 
this is not the case with a single tendril at present 



'The Movements and Habits of Climbing Plants,' p. 36. 



CuAP. V. EPINASTY AND HYPONASTY. 269 

known. The cause of this difference probably lies in 
the fact that the chief service of a pulvinus is to prolong 
the movement of the part thus provided after growth 
has ceased ; and as tendrils or other climbing-organs are 
of use only whilst the plant is increasing in height or 
growing, a pulvinus which served to prolong their move- 
ments would be useless. 

It was shown in the last chapter that the stolons or 
runners of certain plants circumnutate largely, and that 
this movement apparently aids them in finding a passage 
between the crowded stems of adjoining plants. If it 
could be proved that their movements had been modified 
and increased for this special purpose, they ought to 
have been included in the present chapter ; but as the 
amplitude of their revolutions is not so conspicuously 
different from that of ordinary plants, as in the case of 
climbers, we have no evidence on this head. We en- 
counter the same doubt in the case of some plants which 
bury their pods in the ground. This burying process is 
certainly favoured by the circumnutation of the flower- 
peduncle ; but we do not know whether it has been in- 
creased for this special purpose. 

Epin^ AST Y — Hypo nasty. 

The term epinasty is used by De Vries * to express 
greater longitudinal growth along the upper than along 
the lower side of a part, which is thus caused to bend 
downwards ; and by hyponasty is used for the reversed 
process, by which the part is made to bend upwards. 
These actions come into play so frequently .that the use 
of the above two terms is highly convenient. The move- 



*'Arl)eiten des Bot. Inst., in two terms as first used by Schim- 

Wurzhurg,' Heft ii. 1872, p. 223. per, and they have been adopted 

De Vries has slightly modified in this sense by Sachs, 
(p. 252) the meaning of the above 



270 MODIFIED CIRCUMNUTATION. Chap. V. 

ments thus induced result from a modified form of cir- 
cumnutation ; for, as we shall immediately see, an organ 
under the influence of epinasty does not generally move 
in a straight line downwards, or under that of hyponasty 
upwards, but oscillates up and down with some lateral 
movement ; it moves, however, in a preponderant manner 
in one direction. This shows that there is some growth 
on all sides of the part, but more on the upper side in the 
case of epinasty, and more on the lower side in that of 
hyponasty, than on the other sides. At the same time 
there may be in addition, as De Vries insists, increased 
growth on one side due to geotropism, and on another 
side due to heliotropism ; and thus the effects of epinasty 
or of hyponasty may be either increased or lessened. 

He who likes, may speak of ordinary circumnutation 
as being combined with epinasty, hj^ponasty, the effects of 
gravitation, light, &c. ; but it seems to us, from reasons 
hereafter to be given, to be more correct to say that cir- 
cumnutation is modified by these several agencies. We 
will therefore speak of circumnutation, which is always 
in progress, as modified by epinasty, hyponasty, geotro- 
pism, or other agencies, whether internal or external. 

One of the commonest and simplest cases of epinasty is that 
offered by leaves, which at an early age are crowded together 
round the buds, and diverge as they grow older. Sachs first 
remarked that this was due to increased growth along the upper 
side of the petiole and blade ; and De Vries has now shown in 
more detail that the movement is thus caused, aided slightly by 
the weight of the leaf, and resisted as he believes by apoge- 
otropism, at least after the leaf has somewhat diverged. In our 
observations on the circumnutation of leaves, some were selected 
which were rather too young, so that they continued to diverge 
or sink downwards whilst their movements were being traced. 
This may be seen in the diagrams (Figs. 98 and 112, pp. 233 
and 250) representing the circumnutation of the young leaves of 



Chap. V. EPINASTY AND HYPONASTY. 271 

Acanthus mollis and Pelargonium zonale. Similar cases were ob- 
served with Drosera. The movements of a young leaf, only f inch 
in length, of Petunia viola^ea were traced during four days, and 
offer a better instance (Fig. Ill, p. 249), as it diverged during 
the whole of this time in a curiously zigzag line with some of 
the angles sharply acute, and during the latter days plainly cir- 
cumnutated. Some young leaves of about the same age on a 
plant of this Petunia, which had been laid horizontally, and on 
another plant which was left upright, both being kept in com- 
plete darkness, diverged in the same manner for 48 h., and ap- 
parently were not affected by apogeotropism ; though their 
stems were in a state of high tension, for when freed from the 
sticks to which they had been tied, they instantly curled 
upwards. 

The leaves, whilst very young, on the leading shoots of the 
Carnation {Dianthus caryojphyllus) are highly inclined or ver- 
tical; and if the plant is growing vigorously they diverge so 
quickly that they become almost horizontal in a day. But they 
move downwards in a rather oblique line and continue for some 
time afterwards to move in the same direction, in connection, 
we presume, with their spiral arrangement on the stem. The 
course pursued by a young leaf whilst thus obliquely descend- 
ing was traced, and the line was distinctly yet not strongly 
zigzag; the larger angles formed by the successive lines amount- 
ing only to 135°, 154°, and 163°. The subsequent lateral move- 
ment (shown in Fig. 96, p. 231) was strongly zigzag with occa- 
sional circumnutations. The divergence and sinking of the 
young leaves of this plant seem to be very little affected by 
geotropism or heliotropism; for a plant, the leaves of which 
were growing rather slowly (as ascertained by measurement), 
was laid horizontally, and the opposite young leaves diverged 
from one another symmetrically in the usual manner, without 
any upturning in the direction of gravitation or towards the 
light. 

The needle-like leaves of Pinus pinaster form a bundle whilst 
young; afterwards they slowly diverge, so that those on the 
upright shoots become horizontal. The movements of one such 
young leaf was traced during 4-^ days, and the tracing here 
given (Fig. 121) shows that it descended at first in a nearly 



272 



MODIFIED CIRCUMNUTATION. 



Chap. V. 



straight line, but afterwards zigzagged, making one or two 
little loops. The diverging and descending movements of a 

rather older leaf were also 



Fig. 121. 



traced (see former Fig. 113, 
p. 253) : it descended during 
the first day and night in a 
somewhat zigzag line ; it then 
circumnutated round a small 
space and again descended. 
By this time the leaf had near- 
ly assumed its final position, 
and now plainly circumnu- 
tated. As in the case of the 
Carnation, the leaves, whilst 
very young, do not seem to be 
much affected by geotropism 
or heliotropism, for those on 
a young plant laid horizontal- 
ly, and those on another plant 
left upright, both kept in the 
dark, continued to diverge in 
the usual manner without 
bending to either side. 

With Cdbcm scandens, the 
young leaves, as they succes- 
sively diverge from the lead- 
ing shoot which is bent to 
one side, rise up so as to pro- 
ject vertically, and they retain 
this position for some time 
whilst the tendril is revolv- 
ing. The diverging and as- 
cending movements of the 
petiole of one such a leaf, were 
^ traced on a vertical glass un- 

Pinns pinaster : epinastic downward ^j^r a skylight ; and the course 
Tnovcment of a young leaf, pro- . 

duccd by a youn'g plant in a pot, pursued was m most parts 
traced on a vertical glass under a nearly straip-ht, but there were 
skylight, froui 6.45 A.M. June Slid „ , ^ . 

to 10.40 P.M. 6th. two well-marked zigzags (one 




Chap. V. EPINASTY AND HYPONASTY. 273 

of them forming an angle of 112°), and this indicates circum- 
nutation. 

The still closed lobes of a young leaf of Dionaea projected at 
right angles to the petiole, and were in the act of slowly rising. 
A glass filament was attached to the under side of the midrib, 
and its movements were traced on a vertical glass. It circum- 
nutated once in the evening, and on the next day rose, as 
already described (see Fig. 106, p. 340), by a number of acutely 
zigzag lines, closely approaching in character to ellipses. This 
movement no doubt was due to epinasty, aided by apoge- 
otropism, for the closed lobes of a very young leaf on a plant 
which had been placed horizontally, moved into nearly the same 
line with the petiole, as if the plant had stood upright; but at 
the same time the lobes curved laterally upwards, and thus 
occupied an unnatural position, obliquely to the jilane of the 
foliaceous petiole. 

As the hypocotyls and epicotyls of some plants protrude 
from the seed-coats in an arched form, it is doubtful whether 
the arching of these parts, which is invariably present when 
they break through the ground, ought always to be attributed 
to epinasty ; but when they are at first straight and afterwards 
become arched, as often happens, the arching is certainly due 
to epinasty. As long as the arch is surrounded by compact 
earth it must retain its form ; but as soon as it rises above the 
surface, or even before this period if artificially freed from the 
surrounding pressure, it begins to straighten itself, and this no 
doubt is mainly due to hyponasty. The movement of the upper 
and lower half of the arch, and of the crown, was occasionally 
traced; and the course was more or less zigzag, showing 
modified circum nutation. 

With not a few plants, especially climbers, the summit of 
the shoot is hooked, so that the apex points vertically down- 
wards. In seven genera of twining plants * the hooking, or as 
it has been called by Sachs, the nutation of the tip, is mainly 
due to an exaggerated form of circumnutation. That is, the 
growth is so great along one side that it bends the shoot com- 

* 'The Movements and Habits of Climbing Plants,' 2nd edit. p. 13. 



274 MODIFIED CIRCCJMNUTATIOK Chap. V. 

pletely over to the opposite side, thus forming a hook; the 
longitudinal line or zone of growth then travels a little laterally 
round the shoot, and the hook points in a slightly different 
direction, and so onwards until the hook is completely reversed. 
Ultimately it comes back to the point whence it started. This 
was ascertained by painting narrow lines with Indian ink along 
the convex surface of several hooks, and the line was found 
slowly to become at first lateral, then to appear along the con- 
cave surface, and ultimately back again on the convex surface. 
In the case of Lonicera hrachypoda the hooked terminal part of 
the revolving shoot straightens itself periodically, but is never 
reversed ; that is, the periodically increased growth of the con- 
cave side of the hook is sufficient only to straighten it, and not 
to bend it over to the opposite side. The hooking of the tip is 
of service to twining plants by aiding them to catch hold of a 
support, and afterwards by enabling this part to embrace the 
support much more closely than it could otherwise have done at 
first, thus preventing it, as we often observed, from being blown 
away by a strong wind. Whether the advantage thus gained 
by twining plants accounts for their summits being so frequent- 
ly hooked, we do not know, as this structure is not very rare 
with plants which do not climb, and with some climbers (for 
instance, Vitis, Ampelopsis, Cissus, &c.) to whom it does not 
afford any assistance in climbing. 

With respect to those cases in which the tip remains always 
bent or hooked towards the same side, as in the genera just 
named, the most obvious explanation is that the bending is due 
to continued growth in excess along the convex side. Wiesner, 
however, maintains * that in all cases the hooking of the tip is 
the result of its plasticity and weight, — a conclusion which 
from what we have already seen with several climbing plants 
is certainly erroneous. Nevertheless, we fully admit that the 
weight of the part, as well as geotropism, &c., sometimes come 
into play. 

Ampelopsis tricuspidata. — This plant climbs by the aid of 
adhesive tendrils, and the hooked tips of the shoots do not 
appear to be of any service to it. The hooking depends chiefly, 



* 'Sitzb. der k. Akad. der Wissensch.,' Vienna, Jan. 1880, p. 16. 



Chap. V. 



EPINASTY AND HYPONASTY. 



as far as we could ascer- 
tain, on the tip being 
affected by epinasty and 
geotropism ; the lower and 
older parts continually 
straightening themselves 
through hyponasty and 
apogeotropism. We be- 
lieve that the weight of 
the apex is an unimpor- 
tant element, because on 
horizontal or inclined 
shoots the hook is often 
extended horizontally or 
even faces upwards. More- 
over shoots frequently 
form loops instead of 
hooks ; and in this case the 
extreme part, instead of 
hanging vertically down 
as would follow if weight 
was the efficient cause, ex- 
tends horizontally or even 
points upwards. A shoot, 
which terminated in a 
rather open hook, was 
fastened in a highly in- 
clined downward position, 
so that the concave side 
faced upwards, and the re- 
sult was that the apex at 
first curved upwards. This 
apparently was due to 
epinasty and not to apo- 
geotropism, for the apex, 
soon after passing the per- 
pendicular, curved so rap- 
idly downwards that we 
could not doubt that the 



8°aim.I5?^ 



275 



Fig. 122. 



'a/m/4^4fN. 



iO'Sffa^ia 




•/>./;i^* 



I 

I 



S^p 




9°35'a.m^ 



Ampelopsis tricuspidata : hyponastic 
movement of hooked tip of leading 
shoot, traced from 8.10 a.m. July 
13th to 8 A.M 15th. Apex of shoot 
5h inches from the vertical glass. 
Plant illuminated through a sky- 
light. Temp. 17^-19° C. Diagram 
reduced to one-third of original scale. 



276 MODIFIED CIRCUMNUTATION. Chap. V. 

movement was at least aided by geotropism. In the course of 
a few hours the hook was thus converted into a loop with the 
apex of the shoot pointing straight downwards. The longer 
axis of the loop was at first horizontal, but afterwards became 
vertical. During this same time the basal part of the hook 
(and subsequently of the loop) curved itself slowly upwards; 
and this must have been wholly due to apogeotropism in oppo- 
sition to hyponasty. The loop was then fastened upside down, 
so that its basal half would be simultaneously acted on by 
hyponasty (if present) and by apogeotropism ; and now it curved 
itself so greatly upwards in the course of only 4 h. that there 
could hardly be a doubt that both forces were acting together. 
At the same time the loop became open and was thus recon- 
verted into a hook, and this apparently was effected by the 
geotropic movement of the apex in opposition to epinasty. In 
the case of Ampelopsis hederacea, weight plays, as far as we 
could judge, a more important part in the hooking of the tip. 

In order to ascertain whether the shoots of A. tricuspidata 
in straightening themselves under the combined action of hypo- 
nasty and apogeotropism moved in a simple straight course, or 
whether they circumnutated, glass filaments were fixed to the 
crowns of four hooked tips standing in their natural position ; 
and the movements of the filaments were traced on a vertical 
glass. All four tracings resembled each other in a general man- 
ner; but we will give only one (see Fig. 123, p. 375). The 
filament rose at first, which shows that the hook was straight- 
ening itself; it then zigzagged, moving a little to the left be- 
tween 9.35 A.M. and 9 p.m. From this latter hour on the 13th 
to 10.50 A.M. on the following morning (14th) the hook con- 
tinued to straighten itself, and then zigzagged a short dist-mce 
to the right. But from 1 p.m. to 10.40 p.m. on the i4th the 
movement was reversed and the shoot became more hooked. 
During the night, after 10.40 p.m. to 8.15 a.m. on the 15th, the 
hook again opened or straightened itself. By this time the 
glass filament had become so highly inclined that its movements 
could no longer be traced with accuracy ; and by 1.30 p.m. on 
this same day, the crown of the former arch or hook had be- 
come perfectly straight and vertical. There can therefore be 
no doubt that the straightening of the hooked shoot of this 



Chap. V. EPINASTY AND HYPONASTY. 277 

plant is effected by the circumnutation of the arched portion — 
that is, by growth alternating between the upper and lower sur- 
face, but preponderant on the lower surface, with some little 
lateral movement. 

We were enabled to trace the movement of anotlier straight- 
ening shoot for a longer period (owing to its slower growth and 
to its having been placed further from the vertical glass), namely, 
from the early morning on July 13th to late in the evening of 
the 16th. During the whole daytime of the 14th, the hook 
straightened itself very little, but zigzagged and plainly cir- 
cumnutated about nearly the same spot. By the 16th it had 
become nearly straight, and the tracing was no longer accurate, 
yet it was manifest that there was still a considerable amount 
of movement both up and down and laterally ; for the crown 
whilst continuing to straighten itself occasionally became for a 
short time more curved, causing the filament to descend twice 
during the day. 

Smithia Pfundii. — The stiff terminal shoots of this Legu- 
minous water-plant from Africa project so as to make a rectangle 
with the stem below; but this occurs only when the plants are 
growing vigorously, for when kept in a cool place, the summits 
of the stems become straight, as they likewise did at the close 
of the growing season. The direction of the rectangularly bent 
part is independent of the chief source of light. But from ob- 
serving the effects of placing plants in the dark, in which case 
several shoots became in two or three days upright or nearly 
upright, and when brought back into the light again became 
rectangularly curved, we believe that the bending is in part 
due to apheliotropism, apparently somewhat opposed by apoge- 
otropism. On the other hand, from observing the effects of tying 
a shoot downwards, so that the rectangle faced upw^ards, w^e are 
led to believe that the curvature is partly due to epinasty. As 
the rectangularly bent portion of an upright stem grows older, 
the lower part straightens itself; and this is effected through 
hyponasty. He who has read Saclis' recent Essay on the vertical 
and inclined positions of the parts of plants * will see how diffii- 



^- 'Ueber Orthotrope mid Pla- ton des Bot. Inst., in Wiirzburg, 
giotrope Pflanzentheile ; ' ' Arbci- Heft ii. 1879, p. 226. 



278 



MODIFIED CIRCUMNUTATION. 



Chap. V. 



Fig. 123. 



9°p.on. 



e^'a/m.ioH 



'jTaan. 



jra5'a.7n.lZ^ 



yWmJB^Ji 



^lffa.ml2^ 



Smithia Pfundii : hyponastic movement 
of the curved summit of a stem, whilst 
straightening itself, traced from 9 
A.M. July lOtli to 3 P.M. 13th. Apex 
9A inches from the vertical glass. 
Diagram reduced to one - fifth of 
original scale. Plant illuminated 
through skylight. Temp. 17^-19° C. 



cult a subject this is, and 
will feel no surprise at our 
expressing ourselves doubt- 
fully in this and other such 
cases. 

A plant, 20 inches in 
height, was secured to a 
stick close beneath the 
curved summit, which 
formed rather less than a 
rectangle with the stem 
below. The shoot pointed 
away from the observer; 
and a glass filament point- 
ing towards ^he vertical 
glass on which the tracing 
was made, was fixed to 
the convex surface of the 
curved portion. There- 
fore the descending lines in 
the figure represent the 
straightening of the curved 
portion as it grew older. 
The tracing (Fig. 123) was 
begun at 9 a.m. on July 
10th; the filament at first 
moved but little in a zig- 
zag line, but at 2 p.m. it 
began rising and continued 
to do so till 9 P.M. ; and 
this proves that the termi- 
nal portion was being more 
bent downwards. After 9 
P.M. on the 10th an oppo- 
site movement commenced, 
and the curved portion be- 
gan to straighten itself, 
and this continued till 
11.10 A.M. on the 12th, 
but was interrupted by 



Chap. V. BPINASTY AND HYPONASTY. 2Y9 

some small oscillations and zigzags, showing movement in 
different directions. After 11.10 a.m. on the 12th this part 
of the stem, still considerably curved, circumnutated in a con- 
spicuous manner until nearly 3 p.m. on the 13th; but during 
all this time a downward movement of the filament prevailed, 
caused by the continued straightening of the stem. By the 
afternoon of the 13th, the summit, which had originally been 
deflected more than a right angle from the perpendicular, 
had grown so nearly straight that the tracing could no longer 
be continued on the vertical glass. There can therefore be no 
doubt that the straightening of the abruptly curved portion of 
the growing stem of this plant, which appears to be wholly due 
to hyponasty, is the result of modified circumnutation. We 
will only add that a filament was fixed in a different manner 
across the curved summit of another plant, and the same general 
kind of movement was observed. 

Trifolium repens. — In many, but not in all the species of 
Trifolium, as the separate little flowers wither, the sub-pedun- 
cles bend downwards, so as to depend parallel to the upper part 
of the main peduncle. In Tr. subterraneum the main peduncle 
curves downwards for the sake of burying its capsules, and in 
this species the sub-peduncles of the separate flowers bend up- 
wards, so as to occupy the same position relatively to the upper 
part of the main peduncle as in Tr. repens. This fact alone 
would render it probable that the movements of the sub-pedun- 
cles in Tr. repens were independent of geotropism. Neverthe- 
less, to make sure, some flower-heads were tied to little sticks 
upside down and others in a horizontal position; their sub- 
peduncles, however, all quickly curved upwards through the 
action of heliotropism. We therefore protected some flower- 
heads, similarly secured to sticks, from the light, and although 
some of them rotted, many of their sub-peduncles turned very 
slowly from their reversed or from their horizontal positions, 
so as to stand in the normal manner parallel to the upper part 
of the main peduncle. These facts show that the movement is 
independent of geotropism or apheliotropism ; it must there- 
fore be attributed to epinasty, which however is checked, at 
least as long as the flowers are young, by heliotropism. Most 
of the above flowers were never fertilised owing to the exclu- 
19 



280 



MODIFIED CIRCUMNUTATION. 



Chap. V. 



Fig. 124 



\a.m.30f 





eWam.8*Pl 



TrifoUum repens: circuiunutating and 
epinastic movements of the sub- 
peduncle of a single flower, traced 
on a vertical glass under a skylight, 
in A from 11.30 A.M. Aug. 27th to 7 
A.M. 30th ; in B from 7 a.m. Aug. 
30th to a little after 6 p.m. Sept. 8th. 




Chap. T. EPINASTY AND HYPONASTY. 281 

sion of bees; they consequently withered very slowly, and the 
movements of the sub-peduncles were in like manner much 
retarded. 

To ascertain the nature of the movement of the sub-pedun- 
cle, whilst bending downwards, a filament was fixed across the 
summit of the calyx of a not fully expanded and almost upright 
flower, nearly in the centre of the head. The main peduncle 
was secured to a stick close beneath the head. In order to see 
the marks on the glass filament, a few flowers had to be cut 
away on the lower side of the head. The flower under observa- 
tion at first diverged a little from its upright position, so as to 
occupy the open space caused by the removal of the adjoining 
flowers. This required two days, after which time a new trac- 
ing was begun (Fig. 134). In A we see the complex circumnu- 
tating course pursued from 11.30 a.m. Aug. 26th to 7 a.m. on 
the 30th. The pot was then moved a very little to the right, 
and the tracing (B) was continued without interruption from 7 
A.M. Aug. 30th to after 6 p.m. Sept. 8th. It should be observed 
that on most of these days, only a single dot was made each 
morning at the same hour. Whenever the flower was observed 
carefully, as on Aug. 30th and Sept. 5th and 6th, it was found 
to be circurnnutating over a small space. At last, on Sept. 7th, 
it began to bend downwards, and continued to do so until after 
6 P.M. on the 8th, and indeed until the morning of the 9th, 
when its movements could no longer be traced on the vertical 
glass. It was carefully observed during the whole of the 8th, 
and by 10.30 p.m. it had descended to a point lowxr down by 
two-thirds of the length of the figure as here given ; but from 
want of space the tracing has been copied in B, only to a little 
after 6 p.m. On the morning of the 9th the flower was with- 
ered, and the sub-peduncle now stood at an angle of 57° be- 
neath the horizon. If the flower had been fertilised it would 
have withered much sooner, and have moved much more quick- 
ly. We thus see that the sub-peduncle oscillated up and down, 
or circumnutated, during its w^hole downward epinastic course. 

The sub-peduncles of the fertilised and withered flowers of 
Oxalis carnosa likewise bend downwards through epinasty, as 
will be shown in a future chapter; and their downward course 
is strongly zigzag, indicating circumnutation. 



282 MODIFIED CIRCUMNUTATION. Chap. V. 

The number of instances in which various organs 
move through epinasty or hyponasty, often in combina- 
tion with other forces, for the most diversified purposes, 
seems to be inexhaustibly great; and from the several 
cases which have been here given, we may safely infer 
that such movements are due to modified circumnu- 
tation. 



CHAPTER yi. 

Modified Circumnutation : Sleep or Nyctitropic Move- 
ments, THEIR Use: Sleep of Cotyledons. 

Preliminary sketch of the sleep or nyctitropic movements of leaves — 
Presence of pulvini — The lessening of radiation the final cause 
of nyctitropic movements — Manner of trying experiments on 
leaves of Oxalis, Arachis, Cassia, Melilotus, Lotus and Marsilea, 
and on the cotyledons of Mimosa — Concluding remarks on radia- 
tion from leaves — Small differences in the conditions make a great 
difference in the result — Description of the nyctitropic position 
and movements of the cotyledons of various plants — List of species 
— Concluding remarks — Independence of the nyctitropic move- 
ments of the leaves and cotyledons of the same species — Eeasons 
for believing that the movements have been acquired for a special 
purpose. 

The so-called sleep of leaves is so conspicuous a phe- 
nomenon that it was observed as early as the time of 
Pliny ; * and since Linnaeus published his famous Essay, 
' Somnus Plantarum,' it has been the subject of several 
memoirs. Many flowers close at night, and these are 
likewise said to sleep ; but we are not here concerned 
with their movements, for although effected by the same 
mechanism as in the case of young leaves, namely, un- 
equal growth on the opposite sides (as first proved by 
Pfeffer), yet they differ essentially in being excited chiefly 
by changes of temperature instead of light ; and in being 
effected, as far as we can judge, for a different purpose. 



*■ Pfeffer has given a clear and riodischen Bewegungen der Blat- 
interesting sketch of the history torgane,' 1875, p. 163. 
of this subject in his ' Die Pe- 

283 



284 MODIFIED CIRCUMNUTATION. Chap. VI. 

Hardly any one supposes that there is any real analogy 
between the sleep of animals and that of plants,* whether 
of leaves or flowers. It seems, therefore, advisable to 
give a distinct name to the so-called sleep-movements of 
plants. These have also generally been confounded, un- 
der the term "periodic," with the slight daily rise and 
fall of leaves, as described in the fourth chapter; and 
this makes it all the more desirable to give some distinct 
name to sleep - movements. Nyctitropism and nycti- 
tropic, i.e. night- turning, may -be applied both to leaves 
and flowers, and will be occasionally used by us ; but it 
would be best to confine the term to leaves. The leaves 
of some few plants move either upwards or downwards 
when the sun shines intensely on them, and this move- 
ment has sometimes been called diurnal sleep ; but we 
believe it to be of an essentially different nature from 
the nocturnal movement, and it will be briefly consid- 
ered in a future chapter. 

The sleep or nyctitropism of leaves is a large subject, 
and we think that the most convenient plan will be first 
to give a brief account of the position which leaves as- 
sume at night, and of the advantages apparently thus 
gained. Afterwards the more remarkable cases will be 
described in detail, with respect to cotyledons in the 
present chapter, and to leaves in the next chapter. 
Finally, it will be shown that these movements result 
from circumnutation, much modified and regulated by 
the alternations of day and night, or light and dark- 
ness ; but that they are also to a certain extent in- 
herited. 

Leaves, when they go to sleep, move either upwards 
or downwards, or in the case of the leaflets of compound 



*Ch. Eoyer must, however, be Nat.' (5th series), Bot. vol. ix. 
excepted ; see ' Annales des Sc. 1868, p. 378. 



Chap. VI. SLEEP MOVEMENTS. 285 

leaves, forwards, that is, towards the apex of the leaf, %r 
backwards, that is, towards its base ; or, again, they may 
rotate on their own axes without moving either upwards 
or downwards. But in almost every case the plane of 
the blade is so placed as to stand nearly or quite verti- 
cally at night. Therefore the apex, or the base, or either 
lateral edge, may be directed towards the zenith. More- 
over, the upper surface of each leaf, and more especially 
of each leaflet, is often brought into close contact with 
that of the opposite one ; and this is sometimes effected 
by singularly complicated movements. This fact suggests 
that the upper surface requires more protection than the 
lower one. For instance, the terminal leaflet in Trifo- 
lium, after turning up at night so as to stand vertically, 
often continues to bend over until the upper surface is 
directed downwards whilst the lower surface is fully 
exposed to the sky ; and an arched roof is thus formed 
over the two lateral leaflets, which have their upper sur- 
faces pressed closely together. Here we have the unusual 
case of one of the leaflets not standing vertically, or 
almost vertically, at night. 

Considering that leaves in assuming their nyctitropic 
positions often move through an angle of 90° ; that the 
movement is rapid in the evening ; that in some cases, 
as we shall see in the next chapter, it is extraordinarily 
complicated ; that with certain seedlings, old enough to 
bear true leaves, the cotyledons move vertically upwards 
at night, whilst at the same time the leaflets move ver- 
tically downwards; and that in the same genus the 
leaves or cotyledons of some species move upwards, whilst 
those of other species move downwards ;■ — from these and 
other such facts, it is hardly possible to doubt that plants 
must derive some great advantage from such remarkable 
powers of movement. 

The nyctitropic movements of leaves and cotyledons 



286 MODIFIED CmCUMNUTATION. Chap. VI. 

s^e effected in two ways,* firstly, by means of pnlvini 
which become, as Pfeffer has shown, alternately more 
turgescent on opposite sides ; and secondly, by increased 
growth along one side of the petiole or midrib, and then 
on the opposite side, as was first proved by Batalin.f 
But as it has been shown by De Vries J that in these 
latter cases increased growth is preceded by the increased 
turgescence of the cells, the difference between the above 
two means of movement is much diminished, and consists 
chiefly in the turgescence of the cells of a fully developed 
pulvinus, not being followed by growth. When the 
movements of leaves or cotyledons, furnished with a pul- 
vinus and destitute of one, are compared, they are seen 
to be closely similar, and are apparently effected for the 
same purpose. Therefore, with our object in view, it 
does not appear advisable to separate the above two sets 
of cases into two distinct classes. There is, however, one 
important distinction between them, namely, that move- 
ments effected by growth on the alternate sides, are con- 
fined to young growing leaves, whilst those effected by 
means of a pulvinus last for a long time. We have 
already seen well - marked instances of this latter fact 
with cotyledons, and so it is with leaves, as has been 
observed by Pfeffer and by ourselves. The long endur- 
ance of the nycti tropic movements when effected by the 
aid of pulvini indicates, in addition to the evidence 
already advanced, the functional importance of such 
movements to the plant.^ There is another difference 
between the two sets of cases, namely, that there is never, 
or very rarely, any torsion of the leaves, excepting when 
a pulvinus is present ;* but this statement applies only 



•*This distinction,., was first f ' Flora,' 1873, p. 433. 

pointed out (according to Pfeffer, j ' Bot. Zeitung,' 1879, Dec. 

' Die Periodischen Bewegungen lOtli, p. 830. 

der Blattorgane, 1875, p. 161) by # Pfeffer, ' Die Period. Beweg. 

Dassen in 1837. der Blattorgane,' 1875, p. 159. 



Chap. VI. USE OF SLEEP MOVEMENTS. 287 

to periodic and nyctitropic movements, as may be inferred 
from other cases given by Frank.* 

The fact that the leaves of many plants place them- 
selves at night in widely different positions from what 
they hold during the day, but with the one point in com- 
mon, that their upper surfaces avoid facing the zenith, 
often with the additional fact that they come into close 
contact with opposite leaves or leaflets, clearly indicates, 
as it seems to us, that the object gained is the protection 
of the upper surfaces from being chilled at night by ra- 
diation. There is nothing improbable in the upper sur- 
face needing protection more than the lower, as the two 
differ in function and structure. All gardeners know 
that plants suffer from radiation. It is this and not 
cold winds which the peasants of Southern Europe fear 
for their olives, f Seedlings are often protected from 
radiation by a very thin covering of straw ; and fruit- 
trees on walls by a few fir-branches, or even by a fishing- 
net, suspended over them. There is a variety of the 
gooseberry, J the flowers of which from being produced 
before the leaves, are not protected by them from radia- 
tion, and consequently often fail to yield fruit. An ex- 
cellent observer * has remarked that one variety of the 
cherry has the petals of its flowers much curled back- 
ward?, and after a severe frost all the stigmas were 
killed ; whilst at the same time, in another variety with 
incurved petals, the stigmas were not in the least injured. 

This view that the sleep of leaves saves them from 
being chilled at night by radiation, would no doubt have 
occurred to Linnaeus, had the principle of radiation been 

* ' Die Nat Wagerechte Eich- thermometer rises as soon as even 

tung von Pflauzentheilen,' 1870, a fleecy cloud, high in the sky, 

p. 52. passes over the zenith. 

t Martins in 'Bull. Soc. Bot. J ' Loudon's Gardener's Mag.,' 

de France,' torn. xix. 1872. vol. iv. 1828, p. 112. 

Wells, in his famous ' Essay on * Mr. Rivers in ' Gardener's 

Dew,' remarks that an exposed Chron.,' 1866, p. 732. 



288 MODIFIED CIRCUMNUTATIOK Chap. VI. 

then discovered ; for he suggests in many parts of his 
' Somnus Plantarum ' that the position of the leaves at 
night protects the young stems and buds, and often the 
young inflorescence, against cold winds. We are far 
from doubting that an additional advantage may be thus 
gained ; and we have observed with several plants, for 
instance, Desmodium gyrans^ that whilst the blade of 
the leaf sinks vertically down at night, the petiole rises, 
so that the blade has to move through a greater angle in 
order to assume its vertical position than would other- 
wise have been necessary ; but with the result that all 
the leaves on the same plant are crowded together as if 
for mutual protection. 

We doubted at first whether radiation would affect in 
any important manner objects so thin as are many coty- 
ledons and leaves, and more especially affect differently 
their upper and lower surfaces ; for although the temper- 
ature of their upper surfaces would undoubtedly fall 
when freely exposed to a clear sky, yet we thought that 
they would so quickly acquire by conduction the temper- 
ature of the surrounding air, that it could hardly make 
any sensible difference to them, whether they stood hori- 
zontally and radiated into the open sky, or vertically and 
radiated chiefly in a lateral direction towards neighbor- 
ing plants and other objects. We endeavoured, there- 
fore, to ascertain something on this head by preventing 
the leaves of several plants from going to sleep, and by 
exposing to a clear sky when the temperature was be- 
neath the freezing-point, these, as well as the other leaves 
on the same plants which had already assumed their 
nocturnal vertical position. Our experiments show that 
leaves thus compelled to remain horizontal at night, 
suffered much more injury from frost than those which 
were allowed to assume their normal vertical position. 
It may, however, be said that conclusions drawn from 



Chap. VI. USE OF SLEEP MOVEMENTS. 289 

such observations are not applicable to sleeping plants, 
the inhabitants of countries where frosts do not occur. 
But in every country, and at all seasons, leaves must be 
exposed to nocturnal chills through radiation, which 
might be in some degree injurious to them, and which 
they would escape by assuming a vertical ;^osition. 

In our experiments, leaves were prevented from as- 
suming their nyctitropic position, generally by being 
fastened with the finest entomological pins (which did 
not sensibly injure them) to thin sheets of cork sup- 
ported on sticks. But in some instances they were 
fastened down by narrow strips of card, and in others by 
their petioles being passed through slits in the cork. 
The leaves were at first fastened close to the cork, for as 
this is a bad conductor, and as the leaves were not ex- 
posed for long periods, we thought that the cork, which 
had been kept in the house, would very slightly warm 
them ; so that if they were injured by the frost in a 
greater degree than the free vertical leaves, the evidence 
would be so much the stronger that the horizontal posi- 
tion was injurious. But we found that when there was 
any slight difference in the result, which could be de- 
tected only occasionally, the leaves which had been 
fastened closely down suffered rather more than those 
fastened with very long and thin pins, so as to stand from 
■J to j- inch above the cork. This difference in the result, 
which is in itself curious as showing what a very slight 
difference in the conditions influences the amount of 
injury inflicted, may be attributed, as we believe, to the 
surrounding warmer air not circulating freely beneath 
the closely pinned leaves and thus slightly warming 
them. This conclusion is supported by some analogous 
facts hereafter to be given. 

We will now describe in detail the experiments which 
were tried. These were troublesome from our not being 



290 MODIFIED CIRCUMNUTATION. Chap. VI. 

able to predict how much cold the leaves of the several 
species could endure. Many plants had every leaf killed, 
both those which were secured in a horizontal position 
and those which were allowed to sleep — that is, to rise 
up or sink down vertically. Others again had not a 
single leaf irt the least injured, and these had to be 
re - exposed either for a longer time or to a lower 
temperature. 

Oxalis acetosella. — A very large pot, thickly covered with 
between 300 and 400 leaves, had been kept all winter in the 
greenhouse. Seven leaves were pinned horizontally open, and 
were exposed on March 16th for 2 h. to a clear sky, the tempera- 
ture on the surrounding grass being — 4° C. (24° to 25° F.). 
Next morning all seven leaves were found quite killed, so were 
many of the free ones which had previously gone to sleep, and 
about 100 of them, either dead or browned and injured, were 
picked off. Some leaves showed that they had been slightly 
injured by not expanding during the whole of the next day, 
though they afterwards recovered. As all the leaves which 
were pinned open were killed, and only about a third or fourth 
of the others were either killed or injured, we had some little 
evidence that those which were prevented from assuming their 
vertically dependent position suffered most. 

The following night (17th) was clear and almost equally 
cold ( — 3° to — 4° C, on the grass), and the pot was again 
exposed, but this time for only 30 m. Eight leaves had been 
pinned out, and in the morning two of them were dead, whilst 
not a single other leaf on the many plants was even injured. 

On the 23rd the plot was exposed for 1 \\. 30 m., the tem- 
perature on the grass being only — 2° C, and not one leaf was 
injured : the pinned open leaves, however, all stood from ^ to f 
of an inch above the cork. 

On the 24th the pot was again placed on the ground and 
exposed to a clear sky for between 35 m. and 40 m. By a mis- 
take the thermometer was left on an adjoining sun-dial 3 feet 
high, instead of being placed on the grass; it recorded 25*^ to 
26° F. (— 3-3° to — 3-8° C), but when looked at after 1 h. had 



Chap. VI. USE OF SLEEP MOVEMENTS. 291 

fallen to 23° F. (— 5*5° C); so that the pot was perhaps 
exposed to rather a lower temperature than on the two first 
occasions. Eight leaves had been pinned out, some close to the 
cork and some above it, and on the following morning five of 
them (i.e. 63 per cent.) were found killed. By counting a por- 
tion of the leaves we estimated that about 250 had been allowed 
to go to sleep, and of these about 20 were killed (i.e. only 8 per 
cent.), and about 30 injured. 

Considering these cases, there can be no doubt that the 
leaves of this Oxalis, when allowed to assume their normal 
vertically dependent position at night, suffer much less from 
frost than those (23 in number) which had their upper surfaces 
exposed to the zenith. 

Oxalis carnosa. — A plant of this Chilian species was exposed 
for 30 m. to a clear sky, the thermometer on the grass standing 
at — 2° C, with some of its leaves pinned open, and not one 
leaf on the whole bushy plant was in the least injured. On the 
16th of March another plant was similarly exposed for 30 m., 
when the temperature on the grass was only a little lower, viz., 
— 3° to — 4° C. Six of the leaves had been pinned open, and 
next morning five of them were found much browned. The 
plant was a large one, and none of the free leaves, which were 
asleep and depended vertically, were browned, excepting four 
very young ones. But three other leaves, though not browned, 
were in a rather flaccid condition, and retained their nocturnal 
position during the whole of the following day. In this case 
it was obvious that the leaves which were exposed horizontally 
to the zenith suffered most. This same pot was afterwards 
exposed for 35-40 m. on a slightly colder night, and every leaf, 
both the pinned open and the free ones, was killed. It may 
be added that two pots of 0. corniculata (var. Atro purpurea) 
were exposed for 2 h. and 3 h. to a" clear sky with the temp, 
on grass — 2° C, and none of the leaves, whether free or 
pinned open, were at all injured. 

Arachis hypogcea. — Some plants in a pot were exposed at 
night for 30 m. to a clear sky, the temperature on the surround- 
ing grass being — 2° C, and on two nights afterwards they 
were again exposed to the same temperature, but this time 
during 1 h. 30 m. On neither occasion was a single leaf, 



292 MODIFIED CIRCUMNUTATION. Chap. VI. 

whether pinned open or free, injured; and this surprised us 
mucli, considering its native tropical African home. Two 
plants were next exposed (March 16th) for 30 m. to a clear sky, 
the temperature of the surrounding grass being now lower, 
viz., between — 3° and — 4° C, and all four pinned-open leaves 
were killed and blackened. These two plants bore 22 other 
and free leaves (excluding some very young bud-like ones) and 
only two of these were killed and three somewhat injured ; that 
is, 23 per cent, were either killed or injured, whereas all four 
pinned-open leaves were utterly killed. 

On another night two pots with several plants were exposed 
for between 35 m. and 40 m. to a clear sky, and perhaps to a 
rather lower temperature, for a thermometer on a dial, 3 feet 
high, close by stood at — 3*3° to — 3*8° C. In one pot three 
leaves were pinned open, and all were badly injured ; of the 44 
free leaves, 26 were injured, that is, 59 per cent. In the other 
pot 3 leaves were pinned open, and all were killed ; four other 
leaves were prevented from sleeping by narrow strips of stiff 
paper gummed across them, and all were killed; of 24 free 
leaves, 10 were killed, 2 much injured, and 12 unhurt ; that is, 
50 per cent, of the free leaves were either killed or much in- 
jured. Taking the two pots together, we may say that rather 
more than half of the free leaves, which were asleep, were either 
killed or injured, whilst all the ten horizontally extended leaves, 
which had been prevented from going to sleep, were either killed 
or much injured. 

Cassia floribunda. — A bush was exposed at night for 40 m. to 
a clear sky, the temperature on the surrounding grass being 

— 2° C, and not a leaf was injured.* It was again exposed on 
another night for 1 h., when the temperature of the grass was 

— 4° C. ; and now all the leaves on a large bush, whether pinned 
flat open or free, w^ere killed, blackened, and shrivelled, with 
the exception of those on one small branch, low down, which 
was very slightly protected by the leaves on the branches 



* Cassia Isevigata was exposed to injured. But when C. laevigata 

a clear sky for 35 m., and C. was exposed for 1 h., the temp. 

calliantha (a Guiana species) for on the surrounding grass being 

60 m., the temperature on the "between -3° and -4° C, every 

surrounding grass being —2° C, leaf was killed, 
and neither were in the least 



Chap. VI. USE OF SLEEP MOVEMENTS. 293 

above. Another tall bush, with four of its large compound 
leaves pinned out horizontally, was afterwards exposed (temp, 
of surrounding grass exactly the same, viz., — 4° C), but only 
for 30 m. On the following morning every single leaflet on 
these four leaves was dead, with both their upper and lower 
surfaces completely blackened. Of the many free leaves on the 
bush, only seven were blackened, and of these only a single one 
(which was a younger and more tender leaf than any of the 
pinned ones) had both surfaces of the leaflets blackened. The 
contrast in this latter respect was well shown by a free leaf, which 
stood between two pinned-open ones ; for these latter had the 
lower surfaces of their leaflets as black as ink, whilst the inter- 
mediate free leaf, though badly injured, still retained a plain 
tinge of green on the lower surface of the leaflets. This bush 
exhibited in a striking manner the evil effects of the leaves not 
being allowed to assume at night their normal dependent posi- 
tion; for had they all been prevented from doing so, assuredly 
every single leaf on the bush would have been utterly killed by 
this exposure of only 30 m. The leaves whilst sinking down- 
wards in the evening twist round, so that the upper surface is 
turned inwards, and is thus better protected than the outwardly 
turned lower surface. Nevertheless, it was always the upper 
surface which was more blackened than the lower, whenever 
any difference could be perceived between them ; but whether 
this was due to the cells near the upper surface being more 
tender, or merely to their containing more chlorophyll, we do 
not know. 

Melilotus officinalis.— A large pot with many plants, which 
had been kept during the winter in the greenhouse, was exposed 
during 5 h. at night to a slight frost and clear sky. Four 
leaves had been pinned out, and these died after a few days; 
but so did many of the free leaves. Therefore nothing certain 
could be inferred from this trial, though it indicated that the 
horizontally extended leaves suffered most. Another large pot 
with many plants was next exposed for 1 h., the temperature on 
the surrounding grass being lower, viz., — 3° to — 4° C. Ten 
leaves had been pinned out, and the result was striking, for 
on the following morning all these were found much injured or 
killed, and none of the many free leaves on the several plants 



294 MODIFIED CmCUMNUTATION. Chap. YI. 

were at all injured, with the doubtful excejDtion of two or 
three very young ones. 

Melilotus Italica. — Six leaves were pinned out horizontally, 
three with their upper and three with their lower surfaces 
turned to the zenith. The plants were exposed for 5 h. to a 
clear sky, the temperature on ground being about— 1°C. 
Next morning the six pinned-open leaves seemed more injured 
even than the younger and more tender free ones on the same 
branches. The exposure, however, had been too long, for after 
an interval of some days many of the free leaves seemed in 
almost as bad a condition as the pinned-out ones. It was not 
possible to decide whether the leaves with their upper or those 
with their lower surfaces turned to the zenith had suffered 
most. 

Melilotus suaveolens. — Some plants with 8 leaves pinned out 
were exposed to a clear sky during 2 h., the temperature on 
the surrounding grass being — 2° C. Next morning 6 out of 
these 8 leaves were in a flaccid condition. There were about 
150 free leaves on the plant, and none of these were injured, 
except 2 or 3 very young ones. But after two days, the plants 
having been brought back into the greenhouse, the 6 pinned- 
out leaves all recovered. 

Melilotus Taurica. — Several plants were exposed for 5 h. 
during two nights to a clear sky and slight frost, accompanied 
by some wind ; and 5 leaves which had been pinned out suf- 
fered more than those both above and below on the same 
branches which had gone to sleep. Another pot, which had 
likewise been kept in the greenhouse, was exposed for 35-40 m. 
to a clear sky, the temperature of the surrounding grass being 
between — 3° and — 4° C. Nine leaves had been pinned out, 
and all of these were killed. On the same plants there were 
210 free leaves, which had been allowed to go to sleep, and of 
these about 80 were killed, i.e. only 38 per cent. 

Melilotus Petitpierreana. — The plants were exposed to a 
clear sky for 35-40 m. : temperature on surrounding grass — 3° 
to — 4° C. Six leaves had been pinned out so as to stand 
about ^ inch above the cork, and four had been pinned close to 
it. These 10 lea\es were all killed, but the closely pinned ones 
suffered most, as 4 of the G which stood above the cork still 



Chap. VI. USE OF SLEEP MOVEMENTS. 295 

retained small patches of a green colour. A considerable num- 
ber, but not nearly all, of the free leaves, were killed or much 
injured, whereas all the pinned- out ones were killed. 

Melilotiis macrorrJiim. -^The plants were exposed in the same 
manner as in the last case. Six leaves had been pinned out 
horizontally and five of them were killed, that is, 83 per cent. 
We estimated that there were 200 free leaves on the plants, and 
of these about 50 were killed and 20 badly injured, so that about 
35 per cent, of the free leaves were killed or injured. 

Lotus aristata. — Six plants were exposed for nearly 5 h. to 
a clear sky; temperature on surrounding grass — 1'5° C. Four 
leaves had been pinned out horizontally, and 2 of these suf- 
fered more than those above and below on the same branches, 
which had been allowed to go to sleep. It is rather a remark- 
able fact that some j^lants of Lotus Jacohoius^ an inhabitant of 
so hot a country as the Cape Verde Islands, were exposed one 
night to a clear sky, with the temperature of the surrounding 
grass — 2° C, and on a second night for 30 m. with the tem- 
perature of the grass between — 3° and —4° C, and not a 
single leaf, either the pinned-out or free ones, was in the least 
injured. 

Marsilea quadrifoliata. — A large plant of this species — the 
only Cryptogamic plant known to sleep — with some leaves pinned 
open, was exposed for 1 h. 35 m. to a clear sky, the tempera- 
ture on the surrounding ground being — 2° C, and not a sin- 
gle leaf was injured. After an interval of some days the plant 
was again exposed for 1 h. to a clear sky, with the temperature 
on the surrounding ground lower, viz., — 4° C. Six leaves had 
been pinned out horizontally, and all of them were utterly 
killed. The plant had emitted long trailing stems, and these 
had been wrapped round with a blanket, so as to protect them 
from the frozen ground and from radiation ; but a very large 
number of leaves were left freely exposed, which had gone to 
sleep, and of these only 12 were killed. After another inter- 
val, the plant, with 9 leaves pinned out, was again exposed for 
1 h., the temperature on tlie ground being again — 4° C. Six 
of the leaves were killed, and one which did not at first appear 
injured afterwards became streaked with brown. The trailing 
branches, which rested on the frozen ground, had one-half or 
20 



296 MODIFIED CmCUMNUTATION. Cfiap. VI. 

three-quarters of their leaves killed, but of the many other 
leaves on the plant, which alone could be fairly compared with 
the pinned-out ones, none appeared at first sight to have been 
killed, but on careful search 12 were found in this state. After 
another interval, the plant with 9 leaves pinned out, was ex- 
posed for 35-40 m. to a clear sky and to nearly the same, or 
perhaps a rather lower, temperature (for the thermometer by 
an accident had been left on a sun-dial close by), and 8 of 
these leaves were killed. Of the free leaves (those on the trail- 
ing branches not being considered), a good many were killed, 
but their number, compared with the uninjured ones, was small. 
Finally, taking the three trials together, 24 leaves, extended 
horizontally, were exposed to the zenith and to unobstructed 
radiation, and of these 20 were killed and 1 injured; whilst 
a relatively very small proportion of the leaves, which had been 
allowed to go to sleep with their leaflets vertically dependent, 
were killed or injured. 

The cotyledons of several plants were prepared for trial, but 
the weather was mild and we succeeded only in a single in- 
stance in having seedlings of the proper age on nights which 
were clear and cold. The cotyledons of 6 seedlings of Mimosa 
'pudica were fastened open on a cork, and were thus exposed for 
1 h. 45 m. to a clear sky, with the temperature on the surrounding 
ground at 29° F. ; of these, 3 were killed. Two other seedlings, 
after their cotyledons had risen up and had closed together, 
were bent over and fastened so that they stood horizontally 
with the lower surface of one cotyledon fully exposed to the 
zenith, and both were killed. Therefore of the 8 seedlings thus 
tried 5, or more than half, were killed. Seven other seedlings, 
with their cotyledons in their normal nocturnal position, viz., 
vertical and closed, were exposed at the same time, and of these 
only 2 were killed.* Hence it appears, as far as these few trials 
tell anything, that the vertical position at night of the coty- 



* We were surprised that young added that seedlings of the Indian 

seedlings of so tropical a plant as Ca^s'w pt(he>^ceus were exposed for 

Mimosa pudica were able to resist, 1 li. 30 m. to a clear sky, with the 

as well as they did, exposure for temp, on the surrounding ground 

1 hr. 45 m. to a clear sky, the at —2° C, and they were not in 

temperature on tlie surrounding the least injured, 
ground being 29° F. It may be 



Chap. VI. USE OF SLEEP MOVEMENTS. 297 

ledons of Mimosa pudica protects them to a certain degree from 
the evil effects of radiation and cold. 

Concluding Remarlcs on the Radiafdon from Leaves 
at Night. — We exposed on two occasions during the 
summer to a clear sky several pinned-open leaflets of 
Trifolium pratense, which naturally rise at night, and 
of Oxalis puiyurea, which naturally sink at night (the 
plants growing out of doors), and looked at them early 
on several successive mornings, after they had assumed 
their diurnal positions. The difference in the amount of 
dew on the pinned-open leaflets and on those which had 
gone to sleep was generally conspicuous ; the latter being 
sometimes absolutely dry, whilst the leaflets which had 
been horizontal were coated with large beads of dew. 
This shows how much cooler the leaflets fully exposed to 
the zenith must have become, than those which stood 
almost vertically, either upwards or downwards, during 
the night. 

From the several cases .above given, there can be no 
doubt that the position of the leaves at night affects their 
temperature through radiation to such a degree, that 
when exposed to a clear sky during a frost, it is a ques- 
tion of life and death. We may therefore admit as 
highly probable, seeing that their nocturnal position is so 
well adapted to lessen radiation, that the object gained 
by their often complicated sleep movements, is to lessen 
the degree to which they are chilled at night. It should 
be kept in mind that it is especially the upper surface 
which is thus protected, as it is never directed towards 
the zenith, and is often brought into close contact with 
the upper surface of an opposite leaf or leaflet. 

We failed to obtain sufficient evidence, whether the 
better protection of the upper surface has been gained' 
from its being more easily injured than the lower surface, 



298 MODIFIED CIRCUMNUTATION. Chap. VI. 

or from its injury being a greater evil to the plant. That 
there is some difference in constitution between the two 
surfaces is shown by the following cases. Cassia flori- 
hunda was exposed to a clear sky on a sharp frosty night, 
and several leaflets which had assumed their nocturnal 
dependent position with their lower surfaces turned out- 
wards so ^ to be exposed obliquely to the zenith, never- 
theless had these lower surfaces less blackened than the 
upper surfaces which were turned inwards and were in 
close contact with those of the opposite leaflets. Again, a 
pot full of plants of TrifoUum resupinatum^ which had 
been kept in a warm room for three days, was turned out 
of doors (Sept. 21st) on a clear and almost frosty night. 
Next morning ten of the terminal leaflets were examined 
as opaque objects under the microscope. These leaflets, 
in going to sleep, either turn vertically upwards, or more 
commonly bend a little over the lateral leaflets, so that 
their lower surfaces are more exposed to the zenith than 
their upper surfaces. Nevertheless, six of these ten leaflets 
were distinctly yellower on the upper than on the lower 
and more exposed surface. In the remaining four, the 
result was not so plain, but certainly whatever difference 
there was leaned to the side of the upper surface having 
suffered most. 

It has been stated that some of the leaflets experi- 
mented on were fastened close to the cork, and others at 
a height of from ^ to f of an inch above it ; and that 
whenever, after exposure to a frost, any difference could 
be detected in their states, the closely pinned ones had 
suffered most. We attributed this difference to the air, 
not cooled by radiation, having been prevented from cir- 
culating freely beneath the closely pinned leaflets. That 
there was really a difference in the temperature of leaves 
treated in these two different methods, was plainly shown 
on one occasion ; for after the exposure of a pot with 



Chap. VI. USE OF SLEEP MOVEMENTS. 299 

plants of Melilotus dentata for 2 h. to a clear sky (the 
temperature on the surrounding grass being — 2° C), it 
was manifest that more dew had congealed into hoar- 
frost on the closely pinned leaflets, than on those which 
stood horizontally a little above the cork. Again, the 
tips of some few leaflets, which had been pinned close to 
the cork, projected a little beyond the edge, so that the 
air could circulate freely round them. This occurred 
with six leaflets of Oxalis acetosella^ and their tips cer- 
tainly sulfered rather less than the rest of the same 
leaflets ; for on the following morning they were still 
slightly green. The same result followed, even still more 
clearly, in two cases with leaflets of Melilotus officinalis 
which projected a little beyond the cork ; and in two 
other cases some leaflets which were pinned close to the 
cork were injured, whilst other free leaflets on the same 
leaves, which had not space to rotate and assume their 
proper vertical position, were not at all injured. 

Another analogous fact deserves notice : we observed 
on several occasions that a greater number of free leaves 
were injured on the branches which had been kept 
motionless by some of their leaves having been pinned 
to the corks, than on the other branches. This was con- 
spicuously the case with those of Melilotus Petitpierre- 
ana^ but the injured leaves in this instance were not 
actually counted. With Arachis hypogiEa.d, young plant 
with 7 stems bore 22 free leaves, and of these 5 were 
injured by the frost, all of which were on two stems, 
bearing four leaves pinned to the cork-supports. With 
Oxalis carnosa^ 7 free leaves were injured, and every one 
of them belonged to a cluster of leaves, some of which 
had been pinned to the cork. We could account for 
these cases only by supposing that the branches which 
were quite free had been slightly waved about by the 
wind, and that their leaves had thus been a little warmed 



300 MODIFIED CIRCUMNUTATION. Chap. YI. 

by the surrounding warmer air. If we hold our hands 
motionless before a hot fire, and then wave them about, 
we immediately feel relief; and this is evidently an 
analogous, though reversed, case. These several facts — 
in relation to leaves pinned close to or a little above the 
cork-supports — to their tips projecting beyond it — and 
to the leaves on branches kept motionless — seem to us 
curious, as showing how a difference, apparently trifling, 
may determine the greater or less injury of the leaves. 
We may even infer as probable that the less or greater 
destruction during a frost of the leaves on a plant which 
does not sleep, may often depend on the greater or less 
degree of flexibility of their petioles and of the branches 
which bear them. 

Nyctitropic or Sleep Movements of Cotyledons. 

We now come to the descriptive part of our work, 
and will begin with cotyledons, passing on to leaves in 
the next chapter. We have met with only two brief 
notices of cotyledons sleeping. Hofmeister,* after stating 
that the cotyledons of all the observed seedlings of the 
Oaryophyllese (Alsineae and Silenea?) bend upwards at 
night (but to what angle he does not state), remarks that 
those of Stellaria media rise up so as to touch one an- 
other ; they may therefore safely be said to sleep. Sec- 
ondly, according to Eamey,t the cotyledons of Mimosa 
pudica and of CUanthns Dnmpieri rise up almost ver- 
tically at night and approach each other closely. It has 
been shown in a previous chapter that the cotyledons of 
a large number of plants bend a little upwards at night, 
and we here have to meet the difficult question at what 
inclination may they be said to sleep? According to the 
view which we maintain, no movement deserves to be 



-•■ 'Die Lehre YOU der Pflanzcn- t 'Adansonia,' March 10th, 
Z-elle,' 1867, p. 327. 1869. 



Chap. VI. SLEEP OF COTYLEDONS. 301 

called nyctitropic, unless it has been acquired for the 
sake of lessening radiation ; but this could be discovered 
only by a long series of experiments, showing that the 
leaves of each species suffered from this cause, if pre- 
vented from sleeping. We must therefore take an arbi- 
trary limit. If a cotyledon or leaf is inclined at 60° 
above or beneath the horizon, it exposes to the zenith 
about one-half of its area ; consequently the intensity of 
its radiation will be lessened by about half, compared 
with what it would have been if the cotyledon or leaf 
had remained horizontal. This degree of diminution 
certainly would make a great difference to a plant having 
a tender constitution. We will therefore speak of a 
cotyledon and hereafter of a leaf as sleeping, only when 
it lises at night to an angle of about 60°, or to a still 
higher angle, above the horizon, or sinks beneath it to 
the same amount. Not but that a lesser diminution of 
radiation may be advantageous to a plant, as in the case 
of Datura stramonium^ the cotyledons of wdiich rose 
from 31° at noon to 55° at night above the horizon. The 
Swedish tuniip may profit by the area of its leaves being 
reduced at night by about 30 per cent., as estimated by 
Mr. A. S. Wilson ; though in this case the angle through 
which the leaves rose was not observed. On the other 
hand, when the angular rise of cotyledons or of leaves is 
sniall, such as less than 30°, the diminution of radiation 
is so slight that it probably is of no significance to the 
plant in relation to radiation. For instance, the cotyle- 
dons of Geranium Ibcricum rose at night to 27° above 
the horizon, and this would lessen radiation by only 11 
per cent. : those of TAnum. Berendisri rose to 33°, and 
this would lessen radiation by 16 per cent. 

There are, however, some other sources of doubt with 
respect to the sleep of cotyledons. In certain cases, the 
cotyledons whilst young diverge during the day to only 



302 MODIFIED CIRCUMNUTATION. Chap. VI. 

a very moderate extent, so that a small rise at night, 
which we know occurs with the cotyledons of many 
plants, would necessarily cause them to assume a vertical 
or nearly vertical position at night ; and in this case it 
would be rash to infer that the movement was effected 
for any special purpose. On this account we hesitated 
long whether we should introduce several Cucurbitaceous 
plants into the following list ; but from reasons, presently 
to be given, we thought that they had better be at least 
temporarily included. This same source of doubt ap- 
plies in some few other cases ; for at the commencement 
of our observations we did not always attend sufficiently 
to whether the cotyledons stood nearly horizontally in the 
middle of the day. With several seedlings, the cotyle- 
dons assume a highly inclined position at night during 
so short a period of their life, that a doubt naturally 
arises whether this can be of any service to the plant. 
Nevertheless, in most of the cases given in the following 
list, the cotyledons may be as certainly said to sleep as 
may the leaves of any plant. In two cases, namely, with 
the cabbage and radish, the cotyledons of which rise 
almost vertically during the few first nights of their life, 
it was ascertained by placing young seedlings in the 
klinostat, that the upward movement was not due to 
apogeotropism. 

The names of the plants, the cotyledons of which 
stand at night at an angle of at least 60° with the horizon, 
are arranged in the appended list on the same system as 
previously followed. The numbers of the Families, and 
with the Leguminosse the numbers of the Tribes, have 
been added to show how widely the plants in question 
are distributed throughout the dicotyledonous series. A 
few remarks will have to be made about many of the 
plants in the list. In doing so, it will be convenient not 
to follow strictly any systematic order, but to treat of the 



Chap. VI. 



SLEEP OF COTYLEDONS. 



303 



Oxalidse and the Leguminos^ at the close ; for in these 
two Families the cotyledons are generally provided with 
a pulvinus, and their movements endure for a much 
longer time than those of the other plants in the list. 

List of Seedling Plants, the cotyledons of which rise or sink at 
night to an angle of at least 60° above or beneath the horizon. 

Cassia pubescens. 

tora. 

neglecta. 

3 other Brazilian unnamed 



Brassica oleracea. Cruciferse (Fam. 

14). 
napus (as we are informed 

by Prof. Pfeffer). 
Raphanus sativus. Cruciferse. 
Githago segetum. Caryophyllese 

(Fam. 26). 
Stellaria media (according to Hof- 

meister, as quoted). Caryophyl- 

lefee. 
Anoda Wrightii. Malvaceae (Fam. 

36). 
Gossypium (var. Nankin cotton). 

Malvaceae. 
Oxalis rosea. Oxalidse (Fam. 41). 

floribunda. 

articulata. 

Valdiviana. 

sensitiva. 

Geranium rotundifolium. Gera- 

niacese (Fam. 47). 
Trifolium subterraneam. Legu- 

minosse (Fam. 47, Tribe 3). 

srrictum. 

leucantbemum. 

Lotus ornithopopoides. Legumi- 

nossB (Tribe 4). 

peregrin us. 

Jacobseus. 

Clianthus Dampieri. Legumi- 

nosse (Tribe 5) — according to M. 

Eamey. 
Smithia sensitiva. Leguminosse 

(Tribe 6). 
Hgematoxylon Campechianum. 

Leguminosae (Tribe 13) — accord- 
ing to Mr. E. I. Lynch. 
Cassia mimosoides. Leguminosse 

(Tribe 14). 

glauca. 

florida. 

corymbosa. 



?). 



species. 
Bauhinia (sp. 

(Tribe 15). 
Neptunia oleracea. 

(Tribe 20). 
Mimosa pudica. 

(Tribe 21). 
albida. 



Cucurbita ovifera. 

(Fam. 106). 
aurantia. 



Leguminosse 
Leguminosse 
Leguminosse 

Cucurbitacese 



Lagenaria vulgaris. Cucurbitacese. 
Cucumis dudaim. Cucurbitacese. 
Apium petroselinum. Umbel- 

liferse (Fam. 113). 

graveolens. 

Lactuca scariola. Compositse 

(Fam. 122). 
Helianthus annuus(?). Compositse. 
Ipomcea cserulea. Convolvulacese 

(Fam. 151). 

purpurea. 

bona-nox. 

—. coccinea. 

Solanum lycopersicum. Solanese 

(Fam. 157). 
Mimulus, (sp. ?) Scrophularinese 

(Fam. 159) — from information 

given us by Prof. Pfeffer. 
Mirabilis jalapa. Nyctaginese 

(Fam. 177). 

longiflora. 

Beta vulgaris. Polygonese (Fam. 

179). 
Amaranthus caudatus. Amaran- 

thacese (Fam. 180). 
Cannabis sativa (?;. Cannabinese 

(Fam. 195). 



Brassica oleracea (Cruciferae). — It was shown in the first 
chapter that the cotyledons of the common cabhage rise in the 
evening and stand vertically up at night with their petioles in 



304 MODIFIED CIRCUMNUTATION. Chap. VI. 

contact. But as the two cotyledons are of unequal height, they 
frequently interfere a little with each other's movements, the 
shorter one often not standing quite vertically. They awake 
early in the morning; thus at 6.45 a.m. on Nov. 27th, whilst it 
was still dark, the cotyledons, w^hich had been vertical and in 
contact on the previous evening, were reflexed, and thus pre- 
sented a very different appearance. It should be borne in mind 
that seedlings in germinating at the proper season, would not 
be subjected to darkness at this hour in the morning. The 
above amount of movement of the cotyledons is only temporary, 
lasting with plants kept in a warm greenhouse from four to six 
days; how long it would last with seedlings growing out of 
doors we do not know. 

Baphanus sativus. — In the middle of the day the blades of 
the cotyledons of 10 seedlings stood at right angles to their 
hypocotyls, with their petioles a little divergent; at night the 
blades stood vertically, with their bases in contact and with 
their petioles parallel. Next morning, at 6.45 a.m., whilst it 
was still dark, the blades were horizontal. On the following 
night they were much raised, but hardly stood sufficiently ver- 
tical to be said to be asleep, and so it was in a still less degree 
on the third night. Therefore the cotyledons of this plant (kept 
in the greenhouse) go to sleep for even a shorter time than those 
of the cabbage. Similar observations were made, but only dur- 
ing a single day and night, on 13 other seedlings likewise raised 
in the greenhouse, with the same result. 

The petioles of the cotyledons of 11 young seedlings of 
Sinapis nigra were slightly divergent at noon, and the blades 
stood at right angles to the hypocotyls ; at night the petioles 
were in close contact, and the blades considerably raised, with 
their bases in contact, but only a few stood sufficiently upright 
to be called asleep. On the following morning the petioles 
diverged before it was light. The hypocotyl is slightly sen- 
sitive, so that if rubbed with a needle it bends towards the 
rubbed side. In the case of Lepidivm sativum, the petioles of 
the cotyledons of young seedlings diverge during the day and 
converge so as to touch each other during the night, by which 
means the bases of the tripartite blades are brought into con- 
tact ; but the blades are so little raised that they cannot be said 



Chap. VI. SLEEP OP COTYLEDONS. 305 

to sleep. The cotyledons of several other cruciferous plants 
were observed, but they did not rise sufficiently during the 
night to be said to sleep. 

Githogo segetum (CaryophylleaB). — On the first day after the 
cotyledons had burst through the seed-coats, they stood at noon 
at an angle of 75° above the horizon; at night they moved up- 
wards, each through an angle of 15° so as to stand quite verti- 
cal and in contact with one another. On the second day they 
stood at noon at 59° above the horizon, and again at night 
were completely closed, each having risen 31°. On the fourth 
day the cotyledons did not quite close at night. The first and 
succeeding pairs of young true leaves behaved in exactly the 
same manner. We think that the movement in this case may be 
called nyctitropic, though the angle passed through was small. 
The cotyledons are very sensitive to light and will not expand 
if exposed to an extremely dim one. 

Anoda Wrightii (Malvaceae). — The cotyledons whilst mod- 
erately young, and only from % to -3 inch in diameter, sink in 
the evening from their mid day horizontal position to about 35° 
beneath the horizon. But when the same seedlings were older 
and had produced small true leaves, the almost orbicular coty- 
ledons, now '55 inch in diameter, moved vertically downv.ards 
at night. This fact made us suspect that their sinkmg might 
be due merely to their weight; but they were not in the least 
flaccid, and when lifted up sprang back through elasticity into 
their former dependent position. A pot with some old seed- 
lings was turned upside down in the afternoon, before the 
nocturnal fall had commenced, and at night they assumed in 
opposition to their own weight (and to any geotropic action) 
an upwardly directed vertical position. When pots were thus 
reversed, after the evening fall had already commenced, the 
sinking movement appeared to be somewhat disturbed; but all 
their movements were occasionally variable without any appar- 
ent cause. The latter fact, as well as that of the young coty- 
ledons not sinking nearly so much as the older ones, deserves 
notice. Although the movement of the cotyledons endured 
for a long time, no pulvinus was exteriorly visible ; but their 
growth continued for a long time. The cotyledons appear to be 
only slightly heliotropic, though the hypocotyl is strongly sq. 



306 MODIFIED CIRCUMNUTATION. Chap. VI. 

Gossypium arhoreum (?) (var. Nankin cotton) (Malvaceae). — 
The cotyledons behave in nearly the same manner as those of 
the Anoda. On June 15th the cotyledons of tveo seedlings 
were "65 inch in length (measured along the midrib) and stood 
horizontally at noon; at 10 p.m. they occupied the same posi- 
tion and had not fallen at all. On June 23d, the cotyledons of 
one of these seedlings were 1*1 inch in length, and by 10 p.m. 
they had fallen from a horizontal position to 62° beneath the 
horizon. The cotyledons of the other seedling were 1-3 inch 
in length, and a minute true leaf had been formed; they had 
fallen at 10 p.m. to 70*^ beneath the horizon. On June 25th, 
the true leaf of this latter seedling was -9 inch in length, and 
the cotyledons occupied nearly the same position at night. By 
July 9th the cotyledons appeared very old and showed signs of 
withering ; but they stood at noon almost horizontally, and at 
10 P.M. hung down vertically. 

Gossypium herbaceum. — It is remarkable that the cotyledons 
of this species behave differently from those of the last. They 
were observed during 6 weeks from their first development 
until they had grown to a very large size (still appearing fresh 
and green), viz. 2^ inches in breadth. At this age a true leaf 
had been formed, which with its petiole was 2 inches long. 
During the whole of these 6 weeks the cotyledons did not sink 
at night; yet when old their weight was considerable and they 
were borne by much elongated petioles. Seedlings raised from 
some seed sent us from Naples, behaved in the same manner ; 
as did those of a kind cultivated in Alabama and of the Sea- 
island cotton. To what species these three latter forms belong 
we do not know. We could not make out in the case of the 
Naples cotton, that the position of the cotyledons at night 
was influenced by the soil being more or less dry ; care being 
taken that they were not rendered flaccid by being too dry. 
The weight of the large cotyledons of the Alabama and Sea- 
island kinds caused them to hang somewhat downwards, when 
the pots in which they grew were left for a time upside down. 
It should, however, be observed that these three kinds were 
raised in the middle of the winter, which sometimes greatly 
interferes with the proper nyctitropic movements of leaves and 
cotyledons. 



Chap. VI. SLEEP OF COTYLEDONS. 307 

CucurbitacecE. — The cotyledons of Cucurhita aurantia and 
ovifera, and of Lagenaria mdgaris^ stand from the 1st to the 3rd 
day of their lite at about 60° above the horizon, and at night 
rise up so as to become vertical and in close contact with one 
another. With Cucumis dudaim they stood at noon at 45° 
above the horizon, and closed at night. The tips of the coty- 
ledons of all these species are, however, reflexed, so that this 
part is fully exposed to the zenith at night ; and tliis fact is 
opposed to the belief that the movement is of the same nature 
as that of sleeping plants. After the first two or three days the 
cotyledons diverge more during the day and cease to close at 
night. Those of Trichosanthes anguina are somewhat thick and 
fleshy, and did not rise at night; and they could perhaps hardly 
be expected to do so. On the other hand, those of Acantlio- 
sicyos horrida * present nothing in their appearance opposed to 
their moving at night in the same manner as the preceding 
species ; yet they did not rise up in any plain manner. This 
fact leads to the belief that the nocturnal movements of the 
above-named species has been acquired for some special pur- 
pose, which may be to protect the young plumule from radia- 
tion, by the close contact of the whole basal portion of the two 
cotyledons. 

Geranium rotundifoUum (Geraniacese). — A single seedling 
came up accidentally in a pot, and its cotyledons were observed 
to bend perpendicularly downwards during several successive 
nights, having been horizontal at noon. It grew into a fine 
plant but died before flowering : it was sent to Kew and pro- 
nounced to be certainly a Geranium, and in all probability the 
above-named species. This case is remarkable because the 
cotyledons of Q. cinereum, Endressii, Ibericum, Hichardsoni, 
and suhcaulescens were observed during some weeks in the win- 
ter, and they did not sink, whilst those of G. Ibericum rose 27° 
at night. 

Apiuin petroselinum (Umbelliferfe). — A seedling had its coty- 
ledons (Nov, 32ndj almost fully expanded during the day ; by 



■-■■ This plant, from Dammara climber ; it has been described 

Land in S. Africa, is remarkable in 'Transact. Linn. Soc.,' xxvii, 

from being the one known mem- p. 30. 
ber of the Family which is not a 



308 MODIFIED CmCUMNUTATION. Chap. VI. 

8.30 P.M. they had risen considerably, and at 10.30 P.m. were 
almost closed, their tips being only -^~ of an inch apart. On 
the following morning (23rd) the tips were -j^-^% of an inch apart, 
or more than seven times as much. On the next night the coty- 
ledons occupied nearly the same position as before. On the 
morning of the 24th they stood horizontally, and at night were 
60" above the horizon ; and so it was on the night of the 25th. 
But four days afterwards (on the 29th), when the seedlings were 
a week old, the cotyledons had ceased to rise at night to any 
plain degree, 

Apmm graveolens. — The cotyledons at noon were horizontal, 
and at 10 p.m. stood at an angle of 61° above the horizon. 

Lactuca scariola (Compositae). — The cotyledons whilst young 
stood sub-horizontally during the day, and at night rose so as 
to be almost vertical, and some were quite vertical and closed ; 
but this movement ceased when they had grown old and large, 
after an interval of 11 days. 

Helianthus annuus (Compositae). — This case is rather doubt- 
ful; the cotyledons rise at night, and on one occasion they stood 
at 73° above the horizon, so that they might then be said to 
have been asleep. 

Ipomcea cmrulea vel PJiarhitis nil (Convolvulaceae). — The coty- 
ledons behave in nearly the same manner as those of the Anoda 
and Nankin cotton, and like them grow to a large size. Whilst 
young and small, so that their blades were from "5 to '6 of an 
inch in length, measured along the middle to the base of the 
central notch, they remained horizontal both during the middle 
of the day and at night. As they increased in size they began 
to sink more and more in the evening and early night; and 
when they had grown to a length (measured in the above 
manner) of from 1 to 1*25 inch, they sank between 55° and 70° 
beneath the horizon. They acted, however, in this manner only 
when they had been well illuminated during the day. Never- 
theless, the cotyledons have little or no power of bending 
towards a lateral light, although the hypocotyl is strongly helio- 
tropic. They are not provided with a pulvinus, but continue 
to grow for a long time. 

Ipomoea purpurea (vel PharhiHs Mspida).— The cotyledons 
behave in all respects like those of /. ccerulea. A seedling with 



Chap. VI. SLEEP OF COTYLEDONS. 309 

cotyledons -75 inch in length (measured as before) and 1-65 
inch in breadth, having a small true leaf developed, was placed 
at 5.30 P.M. on a klinostat in a darkened box, so that neither 
weight nor geotropism could act on them. At 10 p.m. one coty- 
ledon stood at 77° and the other at 82° beneath the horizon. 
Before being placed in the klinostat they stood at 15° and 29° 
beneath the horizon. The nocturnal position depends chiefly 
on the curvature of the petiole close to the blade, but the whole 
petiole becomes slightly curved downwards. It deserves notice 
that seedlings of this and the last-named species were raised at 
the end of F.ebruary and another lot in the middle of March, 
and the cotyledons in neither case exhibited any nyctitropic 
movement. 

fyomoia lona-nox. — The cotyledons after a few days grow to 
an enormous size, those on a young seedling being SJ inches 
in breadth. They were extended horizontally at noon, and at 
10 P.M. stood at 63° beneath the horizon. Five days after- 
wards they were4| inches in breadth, and at night one stood at 
64° and the other 48° beneath the horizon. Though the blades 
are thin, yet from their great size and from the petioles being 
long, we imagined that their depression at night might be 
determined by their weight ; but when the pot was laid hori- 
zontally, they became curved towards the hypocotyl, which 
movement could not have been in the least aided by their 
w^eight, at the same time they were somewhat twisted upwards 
through apogeotropism. Nevertheless, the weight of the coty- 
ledons is so far influential, that when on another night the pot 
was turned upside down, they were unable to rise and thus to 
assume their proper nocturnal position. 

Ipomma coccinea. — The cotyledons whilst young do not sink 
at night, but when grown a little older, but still only -4 inch in 
length (measured as before) and '82 in breadth, they became 
greatly depressed. In one case they were horizontal at noon, 
and at 10 p.m. one of them stood at 64° and the other at 47° 
beneath the horizon. The blades are thin, and the petioles, 
which become much curved down at night, are short, so that 
here weight can hardly have produced any effect. With all the 
above species of Ipomoea, when the two cotyledons on the same 
seedling were unequally depressed at night, this seemed to 



310 MODIFIED CmCUMNUTATION. Chap. VI. 

depend on the position which they had held during the day 
with reference to the light. 

Solanum lycopersicum (Solanese). — The cotyledons rise so 
much at night as to come nearly in contact. Those of >S'. palina- 
canthum were horizontal at noon, and by 10 p.m. had risen only 
27° 30'; but on the following morning before it was light they 
stood at 59° above the horizon, and in the afternoon of the same 
day were again horizontal. The behaviour of the cotyledons of 
this latter species seems, therefore, to be anomalous. 

Mirabilis jalapa and longiflora (Nyctagineae). — The cotyle- 
dons, which are of unequal size, stand horizontally during the 
middle of the day, and at night rise up vertically and come into 
close contact with one another. But this movenient with M. 
longiflora lasted for only the three first nights. 

Beta vulgaris (Polygoueae). — A large number of seedlings 
were observed on three occasions. During the day the cotyle- 
dons sometimes stood sub-horizontally, but more commonly at 
an angle of about 50° above the horizon, and for the first two 
or three nights they rose up vertically so as to be completely 
closed. During the succeeding one or two nights they rose only 
a little, and afterwards hardly at all. 

Amaranthus caudatus (Amaranthacese). — At noon the cotyle- 
dons of many seedlings, which had just germinated, stood at 
about 45° above the horizon, and at 10.15 p.m. some were nearly 
and others quite closed. On the following morning they were 
again well expanded or open. 

CannaMs sativa (Cannabinese). — We are very doubtful 
whether this plant ought to be here included. The cotyle- 
dons of a large number of seedlings, after being well illumi- 
nated during the day, were curved downwards at night, so that 
the tips of some pointed directly to the ground, but the basal 
part did not appear to be at all depressed. On the following 
morning they were again flat and horizontal. The cotyledons 
of many other seedlings were at the same time not in any way 
affected. Therefore this case seems very different from that of 
ordinary sleep, and probably comes under the head of epinasty, 
as is the case with the leaves of this plant according to Kraus. 
The cot}^ledons are heliotropic, and so is the hypocotyl in a 
still stronofer dee-ree. 



Chap. VI. SLEEP OF COTYLEDONS. 311 

Oxalis. — We now come to cotyledons provided with a pul- 
vinus, all of which are remarkable from the continuance of the 
nocturnal movements during several days or even weeks, and 
apparently after growth has ceased. The cotyledons of 0. 
rosea, Jioribunda and articulata sink vertically down at night 
and clasp the upfter part of the hypocotyl. Those of 0. Valdi- 
mana and sensitiva, on the contrary, rise vertically up, so that 
their upper surfaces come into close contact; and after the 
young leaves are developed these are clasped by the cotyle- 
dons. As in the daytime they stand horizontally, or are even 
a little deflected beneath the horizon, they move in the evening 
through an angle of at least 90°. Their complicated circum- 
nutating movements during the day have been described in the 
first chapter. The experiment was a superfluous one, but pots 
with seedlings of 0. rosea and Jloribu7ida were turned upside 
down, as soon as the cotyledons began to show any signs of 
sleep, and this made no difference in their movements. 

Leguminosce. — It may be seen in our list that the cotyledons 
of several species in nine genera, widely distributed through- 
out the Family, sleep at night; and this probably is the case 
with many others. The cotyledons of all these species are pro- 
vided with a pulvinus; and the movement in all is continued 
during many days or weeks. In Cassia the cotyledons of the 
ten species in the list rise up vertically at night and come into 
close contact with one another. We observed that those of C. 
JJorida opened in the morning rather later than those of C. 
glauca and pube^cens. The movement is exactly the same in 
C. mimosoides as in the other species, though its subsequently 
developed leaves sleep in a different manner. The cotyledons 
of an eleventh species, namely, C. nodosa, are thick and fleshy, 
and do not rise up at night. The circumnutation of the cotyle- 
dons during the day of C. tora has been described in the first 
chapter. Although the cotyledons of Smitlna sensitiva rose 
from a horizontal position in the middle of the day to a ver- 
tical one at night, those of 8. Pfundii, which are thick and 
fleshy, did not sleep. When Mimosa pudica and alUda have 
been kept at a sufficiently high temperature during the day, the 
cotyledons come into close contact at night; otherwise they 
merely ri£,e up almost vertically. The circumnutation of those 
21 



312 MODIFIED CIRCUMXUTATION. Chap. VI. 

of M. pudica has been described. The cotyledons of a Bau- 
hinia from St. Catharina in Brazil stood during the day at an 
angle of about 50° above the horizon, and at night rose to 77°; 
but it is probable that they would have closed completely, if 
the seedlings had been kept in a warmer place. 

Lotus. — In three species of Lotus the cotyledons were ob- 
served to sleep. Those of L. Jacobceus present the singular case 
of not rising at night in any conspicuous manner for the first 
5 or 6 days of their life, and the pulvinus is not well developed 
at this period. Afterwards the sleeping movement is well dis- 
played, though to a variable degree, and is long continued. 
We shall hereafter meet with a nearly parallel case with the 
leaves of ISida rliomtifolia. The cotyledons of L. Qebelii are 
only slightly raised at night, and differ much in this respect 
from the three species in our list. 

TrifoUum. — The germination of 21 species was observed. 
In most of them the cotyledons rise hardly at all, or only 
slightly, at night; but those of T. glomeratum, striatum and 
incarnatum rose from 45° to 55° above the horizon. With T. 
subterraneum^ leucantJiemum and strictum^ they stood up verti- 
cally; and with T. strictum the rising movement is accompa- 
nied, as we shall see, by another movement, which makes us 
believe that the rising is truly nyctitropic. We did not care- 
fully examine the cotyledons of all the species for a pulvinus, 
but this organ was distinctly present in those of T. sxibterra- 
neum and strictum ; whilst there was no trace of a pulvinus in 
some species, for instance, in T. resupinatum, the cotyledons of 
which do not rise at night. 

TrifoUum si^terraneum. — The blades of the cotyledons on 
the first day after germination (Nov. 21st) were not fully ex- 
panded, being inclined at about 35° above the horizon; at night 
they rose to about 75°. Two days afterwards the blades at 
noon were horizontal, with the petioles highly inclined up- 
wards; and it is remarkable that the nocturnal movement is 
almost wholly confined to the blades, being effected by the pul- 
vinus at their bases; whilst the petioles retain day and night 
nearly the same inclination. On this night (Nov. 23rd), and 
for some few succeeding nights, the blades rose from a hori- 
zontal into a vertical position, and then became bowed inwards 



Chap. VI. SLEEP OF COTYLEDONS. 313 

at about an average angle of 10° ; so that they had passed 
through an angle of 100°. Theu' tips now almost touched one 
another, their bases being slightly divergent. The two blades 
thus formed a highly inclined roof over the axis of the seed- 
ling. This movement is the same as that of the terminal leaflet 
of the tripartite leaves of many species of Trifolium. After an 
interval of 8 days (Nov. 29th) the blades were horizontal dur- 
ing the day, and vertical at night, and now they were no longer 
bowed inwards. They continued to move in the same manner 
for the following two months, by which time they had increased 
greatly in size, their petioles being no less than -8 of an inch in 
length, and two true leaves had-by this time been developed. 

Trifolium strcctum. — On the first day after germination the 
cotyledons, which are provided with a pulvinus, stood at noon 
horizontally, and at night rose to only about 45° above the 
horizon. Four days afterwards the seedlings were again ob- 
served at night, and now the blades stood vertically and were 
in contact, excepting the tips, whic^i were much deflexed, so 
that they faced the zenith. At this age the petioles are curved 
upwards, and at night, when the bases of the blades are in con- 
tact, the two petioles together form a vertical ring surrounding 
the plumule. The cotyledons continued to act in nearly the 
same manner for 8 or 10 days from the period of germination; 
but the petioles had by this time become straight and had in- 
creased much in length. After from 12 to 14 days the first 
simple true leaf was formed, and during the ensuing fortnight 
a remarkable movement was repeatedly observed. At I. (Fig. 
125) we have a sketch, made in the middle of the day, of a 
seedling about a fortnight old. The two cotyledons, of which 
Be is the right, and Lc the left one, stand directly opposite one 
another, and the first true leaf (F) projects at right angles to 
them. At night (see II. and III.) the right cotyledon (Ec) is 
greatly raised, but is not otherwise changed in position. The 
left cotyledon (Lc) is likewise raised, but it is also twisted, so 
that its blade, instead of exactly facing the opposite one, now 
stands at nearly right angles to it. This nocturnal twisting 
movement is effected not by means of the pulvinus, but by the 
twisting of the whole length of the petiole, as could be seen by 
the curved line of its upper concave surface. At the same time 



3U 



MODIFIED CIRCUMNUTATION. Chap. YI. 



the true leaf (F) rises up, so as to stand vertically, or it even 
passes the vertical and is inclined a little inwards. It also 
twists a little, by which means the upper sui-face of its blade 




TrifoUum stridum : diurnal and nocturnal positions of the two cotyle- 
dons and of the first leaf. I. Seedling viewed obliquely from 
above, during the day : Ec. right cotyledon ; Lc. left cotyledon ; 
F, first true leaf. II. A rather younger seedling, viewed at night ; 
Ec, right cotyledon i-aised. but its position not otherwise changed ; 
Lc, left, cotyledon raised and laterally twisted : F, first leaf raised 
and twisted so as to face the left twisted cotyledon. III. Same 
seedling viewed at night from the opposite side. The back of the 
first leaf, F, is here shown instead of the front, as in II. 



fronts, and almost comes into contact with, the upper surface 
of the twisted left cotyledon. This seems to be the object 
gained by these singular movements. Altogether 20 seedlings 
were examined on successive nights, and in 19 of them it was 
the left cotyledon alone which became twisted, with the true 
leaf always so twisted that its upper surface approached closely 
and fronted that of the left cotyledon. In only one instance 
was the right cotyledon twisted, with the true leaf twisted 
towards it ; but this seedling was in an abnormal condition, as 
the left cotyledon did not rise up properly at night. This 
whole case is remarkable, as with the cotyledons of no other 
plant have we seen any nocturnal movement except vertically 
upwards or downwards. It is the more remarkable, because 
we shall meet with an analogous case in the leaves of the 
allied genus Melilotus, in which the terminal leaflet rotates 
at night so as to present one edge to the zenith and at the 
same time bends to one side, so that its upper surface comes 
into contact with that of one of the two now vertical lateral 
leaflets. 



Chap. VI. SLEEP OF COTYLEDONS. 315 

Concluding Remarlcs on the Nyctitropic Movements 
of Cotyledons. — The sleep of cotyledons (though this is a 
subject which has been little attended to), seems to be 
a more common phenomenon than that of leaves. We 
observed the position of the cotyledons during the day 
and night in 153 genera, widely distributed throughout 
the dicotyledonous series, but otherwise selected almost 
by hazard ; and one or more species in 26 of these genera 
placed their cotyledons at night so as to stand vertically 
or almost vertically, having generally moved through an 
angle of at least 60°. If we lay on one side the Legu- 
minosaB, the cotyledons of which are particularly liable 
to sleep, 140 genera remain ; and out of these, the coty- 
ledons of at least one species in 19 genera slept. Now 
if we were to select by hazard 140 genera, excluding the 
Leguminosae, and observed their leaves at night, assur- 
edly not nearly so many as 19 would be found to in- 
clude sleeping species. We here refer exclusively to the 
plants observed by ourselves. 

In our entire list of seedlings, there are 30 genera, 
belonging to 16 Families, the cotyledons of which in 
some of the species rise or sink in the evening or early 
night, so as to stand at least 60° above or beneath the 
horizon. In a large majority of the genera, namely, 24, 
the movement is a rising one ; so that the same direction 
prevails in these nyctitropic movements as in the lesser 
periodic ones described in the second chapter. The 
cotyledons move downwards during the early part of 
the night in only 6 of the genera; and in one of them. 
Cannabis, the curving down of the tip is probably due to 
epinasty, as Kraus believes to be the case with the leaves. 
The downward movement to the amount of 90 is very 
decided in OxalisVcddiviana and sensitiva, and in Gera- 
nium rotundifolium. It is a remarkable fact that with 
Anoda Wriyhtii, one species of Gossypium and at least 



316 MODIFIED CIRCUMNUTATION". Chap. VI. 

3 species of Ipomoea, the cotyledons whilst young and 
light sink at night very little or not at all; although 
this movement becomes well pronounced as soon as they 
have grown large and heavy. Although the downward 
movement cannot be attributed to the weight of the 
cotyledons in the several cases which were investigated, 
namely, in those of the Anoda, Ipomcea purpurea and 
hona-nox^ nor in that of /. coccinea^ yet bearing in mind 
that cotyledons are continually circumnutating, a slight 
cause might at first have determined whether the great 
nocturnal movement should be upwards or downwards. 
We may therefore suspect that in some aboriginal mem- 
ber of the groups in question, the weight of the coty- 
ledons first determined the downward direction. The 
fact of the cotyledons of these species not sinking down 
much whilst they are young and tender, seems opposed 
to the belief that the greater movement when they are 
grown older, has been acquired for the sake of protect- 
ing them from radiation at night; but then we should 
remember that there are many plants, the leaves of which 
sleep, whilst the cotyledons do not ; and if in some cases 
the leaves are protected from cold at night whilst the 
cotyledons are not protected, so in other cases it may be 
of more importance to the species that the nearly full- 
grown cotyledons should be better protected than the 
young ones. 

In all the species of Oxalis observed by us, the coty- 
ledons are provided with pulvini; but this organ has 
become more or less rudimentary in 0. cornicuJata^ and 
the amount of upward movement of its cot3dedon3 at 
night is very variable, but is never enough to be called 
sleep. We omitted to ascertain whether the cotyledons 
of Gerankim rotundifoHi(m possess pulvini. In the 
Leguminosae all the cotyledons which sleep, as far as we 
have seen, are provided with pulvini. But with Lotus 



Chap. VI. SLEEP OF COTYLEDONS. 317 

JacoicBUS^ these are not fully developed during the first 
few days of the life of the seedling, and the cotyledons 
do not then rise much at night. With Tri folium stric- 
tum the blades of the cotyledons rise at night by the aid 
of their pulvini; whilst the petiole of one cotyledon 
twists half-round at the same time, independently of its 
pulvinus. 

As a general rule, cotyledons which are provided with 
pnlvini continue to rise or sink at night during a much 
longer period than those destitute of this organ. In 
this latter case the movement no doubt depends on 
alternately greater growth on the u^oper and lower side 
of the petiole, or of the blade, or of both, preceded prob- 
ably by the increased turgescence of the growing cells. 
Such movements generally last for a very short period — 
for instance, with Brassica and Githago for 4 or 5 nights, 
with Beta for 2 or 3, and with Raphanus for only a single 
night. There are, however, some strong exceptions to 
this rule, as the cotyledons of Gossypium, Anoda and 
Ipomoea do not possess pulvini, yet continue to move 
and to grow for a long time. We thought at first that 
when the movem.ent lasted for only 2 or 3 nights, it 
could hardly be of any service to the plant, and hardly 
deserved to be called sleep ; but as many quickly-growing 
leaves sleep for only a few nights, and as cotyledons are 
rapidly developed and soon complete their growth, this 
doubt now seems to us not well-founded, more especially 
as these movements are in many instances so strongly 
pronounced. We may here mention another point of 
similarity between sleeping leaves and cotyledons, name- 
ly, that some of the latter (for instance, those of Cassia 
and Githago) are easily affected by the absence of light ; 
and they then either close, or if closed do not open ; 
whereas others (as witli the cotyledons of Oxalis) are 
very little affected by light. In the next chapter it will 



318 MODIFIED CmCUMNUTATION. Chap. VI. 

be shown that the nyctitropic movements both of coty- 
ledons and leaves consist of a modified form of circum- 
nutation. 

As in the Leguminosse and Oxalidse, the leaves and 
the cotyledons of the same species generally sleep, the 
idea at first naturally occurred to us, that the sleep of 
the cotyledons was merely an early development of a 
habit proper to a more advanced stage of life. ' But no 
such explanation can be admitted, although there seems 
to be some connection, as might have been expected, 
between the two sets of cases. For the leaves of many 
plants sleep, whilst their- cotyledons do not do so — of 
which fact Desmodium gyrans offers a good instance, as 
likewise do three species of Mcotiana observed by us; 
also Sida rhomlifolia^ AbtUilon Darwinii, and Clieno- 
podium album. On the other hand, the cotyledons of 
some plants sleep and not the leaves, as with the species 
of Beta, Brassica, Geranium, Apium, Solanum, and Mira- 
bilis, named in our list. Still more striking is the fact 
that, in the same genus, the leaves of several or of all 
the species may sleep, but the cotyledons of only some 
of them, as occurs with Trifolium, Lotus, Gossypium, 
and partially with Oxalis. Again, when both the coty- 
ledons and the leaves of the same plant sleep, their 
movements may be of a widely dissimilar nature ; thus 
with Cassia the cotyledons rise vertically up at night, 
whilst their leaves sink down and twist round so as to 
turn their lower surfaces outwards. With seedlings of 
Oxcdis Valdiviana, having 2 or 3 well-developed leaves, 
it was a curious spectacle to behold at night each leaflet 
folded inwards and hanging perpendicularly downwards, 
whilst at the same time and on the same plant the coty- 
ledons stood vertically upwards. 

These several facts, showing the independence of the 
nocturnal movements of the leaves and cotyledons on the 



Chap. VI. SLEEP OF COTYLEDONS. 319 

same plant, and on plants belonging to the same genus, 
lead to the belief that the cotyledons have acquired their 
power of movement for some special purpose. Other 
facts lead to the same conclusion, such as the presence 
of pulvini, by the aid of which the nocturnal movement 
is continued during some weeks. In Oxalis the cotyle- 
dons of some species move vertically upwards, and of 
others vertically downwards at night ; but this great dif- 
ference within the same natural genus is not so surpris- 
ing as it may at first appear, seeing that the cotyledons 
of all the species are continually oscillating up and down 
during the day, so that a small cause might determine 
whether they should rise or sink at night. Again, the 
peculiar nocturnal movement of the left-hand cotyledon 
of Trifolium strictum, in combination with that of the 
first true leaf. Lastly, the wide distribution in the di- 
cotyledonous series of plants with cotyledons which sleep. 
Eeflecting on these several facts, our conclusion seems 
justified, that the nyctitropic movements of cotyledons, 
by which the blade is made to stand either vertically or 
almost vertically upwards or downwards at night, has 
been acquired, at least in most cases, for some special 
purpose; nor can we doubt that this purpose is the pro- 
tection of the upper surface of the blade, and perhaps of 
the central bud or plumule, from radiation at night. 



CHAPTER VII. 

Modified Circumnutation: Nyctitropic or Sleep Move- 
ments OF Leaves, 

Conditions necessary for these movements — List of Genera and Fami- 
lies, which include sleeping plants — Description of the movements 
in the several Genera — Oxalis : leaflets folded at night — Averrhoa : 
rapid movements of the leaflets — Porlieria : leaflets close when 
plant kept very dry — Tropseolum ; leaves do not sleep unless well 
illuminated during day — Lupinus : various modes of sleeping — 
Melilotus : singular movements of terminal leaflet — Trifolium — 
Desmodium : rudimentary lateral leaflets, movements of, not de- 
veloped on young plants, state of their pulvini — Cassia : complex 
movements of the leaflets — Bauhinia : leaves folded at night — 
Mimosa pudica : compounded movements of leaves, effect of dark- 
ness — Mimosa albida, reduced leaflets of— Schrankia : downward 
movement of the pinnse — Marsilea : the only cryptogam known to 
sleep— Concluding remarks and summary — Nyctitropism consists 
of modified circumnutation, regulated by the alternations of light 
and darkness — Shape of first true leaves. 

We now come to the nyctitropic or sleep movements 
of leaves. It should be remembered that we confine this 
term to leaves which place their blades at night either in 
a vertical position or not more than 30° from the verti- 
cal, — that is, at least 60° above or beneath the horizon. 
In some few cases this is effected by the rotation of the 
blade, the petiole not being either raised or lowered to 
any considerable extent. Tiie limit of 30° from the ver- 
tical is obviously an arbitrary one, and has been selected 
for reasons previously assigned, namely, that when the 
blade approaches the perpendicular as nearly as this, 
only half as much of the surface is exposed at night to 
the zenith and to free radiation as when the blade is 



Chap. YII. SLEEP OF LEAVES. 321 

horizontaL Nevertheless, in a few instances, leaves 
which seem to be prevented by their structure from 
moving to so great an extent as 60° above or beneath 
the horizon, have been included amongst sleeping plants. 
It should be premised that the nyctitropic move- 
ments of leaves are easily aSected by the conditions to 
which the plants have been subjected. If the ground is 
kept too dry, the movements are much delayed or fail : 
according to Dassen,* even if the air is very dry the 
leaves of Impatiens and Malva are rendered motionless. 
Carl Kraus has also lately insisted f on the great influ- 
ence which the quantity of water absorbed has on the 
periodic movements of leaves ; and he believes that this 
cause chiefly determines the variable amount of sinking 
of the leaves of Polygonum convolvulus at night ; and if 
so, their movements are not in our sense strictly nycti- 
tropic. Plants in order to sleep must have been exposed 
to a proper temperature : Erytlirina Crista-galli^ out of 
doors and nailed against a wall, seemed in fairly good 
health, but the leaflets did not sleep, whilst those on 
another plant kept in a warm greenhouse were all ver- 
tically dependent at night. In a kitchen-garden the 
leaflets of Phaseolus vulgaris did not sleep during the 
early part of the summer. Ch. Eoyer says, J referring I 
suppose to the native plants in France, that they do not 
sleep when the temperature is below 5° 0. or 41° F. In 
the case of several sleeping plants, viz., species of Tro- 
paeolum, Lupinus, Ipomoea, Abutilon, Siegesbeckia, and 
probably other genera, it is indispensable that the leaves 
should be well illuminated during the day in order that 



"•'Dassen, ' Tijdschrift vor Na- Bot.' (^ih serips\ ix 186S p 345 

tiirlijke Gesch. en Physiologic,' f ' Beitnigc zur Kcnntni-s der 

1837. vol. IV. p. 106. See also Epwegunfjen,' Sic,., in 'Flora,' 

Ch. Royer on the importance of 1879, pp. 42, 43. 67. &c. 

a propf^r state of tursrescencc of ± '.Annal. rlcs Sc. N^^ Bot' 

the cells, in ' Annal. des So. Nat. (5th series), ix. 1868, p. 366. 



322 MODIFIED CIRCUMNUTATION. Chap. VII. 

they may assume at night a vertical position ; and it was 
probably owing to this cause that seedlings of CJieno- 
podium album and Siegesheckia orientalis^ raised by us 
during the middle of the winter, though kept at a proper 
temperature, did not sleep. Lastly, violent agitation by 
a strong wind, during a few minutes, of the leaves of 
Maranta arundinacea (which previously had not been 
disturbed in the hot-house), prevented their sleeping 
during the two next nights. 

We will now give our observations on sleeping plants, 
made in the manner described in the Introduction. 
The stem of the plant was always secured (when not 
stated to the contrary) close to the base of the leaf, the 
movements of which were being observed, so as to pre- 
vent the stem from circumnutating. As the tracings 
were made on a vertical glass in front of the plant, it 
was obviously impossible to trace its course as soon as 
the leaf became in the evening greatly inclined either 
upwards or downwards ; it must therefore be understood 
that the broken lines in the diagrams, which represent 
the evening and nocturnal courses, ought always to be 
prolonged to a much greater distance, either upwards or 
downwards, than appears in them. The conclusions 
which m-ay be deduced from our observations will be 
given near the end of this chapter. 

In the following list all the genera which include 
sleeping plants are given, as far as known to us. The 
same arrangement is followed as in former cases, and 
the number of the Family is appended. This list pos- 
sesses some interest, as it shows that the habit of sleeping 
is common to some few plants throughout the whole vas- 
cular series. The greater number of the genera in the 
list have been observed by ourselves with more or less 
care; but several are ^iven on the authority of others 
(whose names are appended in the list), and about these 



Chap. VII. 



SLEEP OF LEAVES. 



32; 



we have nothing more to say. ]N"o doubt the list is very 
imperfect, and several genera might have been added 
from the " Somnus Plantarum " by Linnaeus ; but we 
could not judge, in some of his cases, whether the blades 
occupied at night a nearly vertical position. ' He refers 
to some plants as sleeping, for instance, Lathyrus odora- 
tus Siudi Yicia faba^ in which we could observe no move- 
ment deserving to be called sleep, and as no one can doubt 
the accuracy of Linnaeus, we are left in doubt. 

List of Genera^ including species the leaves of which sleep. 



Class I. DICOTYLEDONS. 



Sub-class I. Angiosperms. 



Genus. 
Githago, 

Stellaria (Batalin). 
Portulaca (Ch. \ 

Eoyer). j 

Sida. 
Abutilon. 
Malva (Linnaeus (_ 

and Pfeffer). S 
Hibiscus (Linnseusj. 
Anoda. 
Gossypium. 
Ayenia (Linnaeus). 
Triumfetta (Lin- ] 

nffius). f 

Linnm (Batalin). 
Oxalis. 
Averrhoa. 
Porlieria. 
Gniacnm. 
Impatiens 

na?ns. 

Batalin). 
Tropaeolnm. 
Crotolaria (Thisel 

ton Dyer). 
Lnpinus, 
CvtisiiP. 
Trieonella. 
Medica^o. 
Melilotns. 
Tri folium. 
Secnrigera. 
Lotufs. 
P.enralea. 
Amoroba 

cbiirtre). 
Dflelea. 
Indisofera 
Tephrosia. 
Wistaria. 



(Lin- 
Pfeffer, 



(Dn- I 



Family. 
Caryophylleae (26). 

Portulaceae (27). 
MalvaceiB (36). 



Sterculaceae (37). 

Tiliaceae (38). 

Lineae (39). 
Oxalidse (41). 

Zygophylleae (45). 



Balsaminese (48). 

TropjEoleae (49). 
Legurainosse (75) 
Tribe II. 



Tr. III. 



Tr. IV. 
Tr. V. 



Sub-class I. Angiosperms {continued). 



Genus. 
Eobinia. 

Spbserophysa. 

Colutea. 

Astragalus. 

Glycyrrhiza. 

Coronilla. 

Hedysarum. 

Onobrycbis. 

Smithia. 

Arachis. 

Desmodium. 

Ll^rania. 

Vicia. 

Centrosema. 

Ampbicarpsea. 

Glycine. 

Erythrina. 

Apios. 

Phaseolus. 

Sophora. 

Citsalpinia. 

Hsematoxylon. 

Gleditscbia (Du- ) 

cbartre. j 

Poinciana. 
Cassia. 
Baubinia. 
Tamarindus. 
Adenantbera. 
Prosopis. 
Neptunia. 
Mimosa. 
Scbrankia. 
Acacia. 
Albizzia. 

Melaleuca (Boucbe). 
JEnotbera (Lin- { 

na^up). \ 

Passiflora. 
Siegesbeckia. 

Ipomoea. 



Farnily. 
j Leguminosae (75) 
/ Tr. V. 



Tr. VI. 



Tr. VII. 
Tr. VIII. 



Tr. X. 
Tr. XIIL 



Tr. XIV. 
Tr. XV. 
Tr. XVt. 
Tr. XX. 



" Tr. XXIL 
" Tr. XXIIL 
Myrtaceae (94). 

Onagrarieae (100). 

Passifloraceac (105). 
Compositap (122^. 

J Convolvulaceae 

( USD. 



324 



MODIFIED CIRCUMNUTATION. Chap. VII. 



Class I. DICOTYLEDONS {continued). 
Sub-class I. Angiospekms {continued). 



Genus. 
Nicotiana. 
Mirabilis. 
Pol3'gonuni (Ba- 
talin). 

Amaranthus. 

Chenopodium. 
Pimeliu (Bouche). 
Euphorbia. 
Phyllanthiis (Pfef- 
fer). 



Family. 
Solanete (157). 
Njctagiuese (177). 

Polygouese (179). 

j Amaranthacete 

I (180). 

Chenopodiese (181). 
Thymeteae (188). 
EuphorbiacejB (.202). 



Sub-class II. Gymnosperms. 
Genus. Family. 

Abies (Chatin). | 



Clas 



Thalia. 
Maranta. 
Colocasia. 
fcjtrephium, 



II. MONOCOTYLEDONS. 

Cannacese (21). 



Aroidese (30). 
(iraminese, (55). 



Class IIL ACOTYLEDONS. 

Marsilea. | Marsileacese (4). 



Qithago segetum (Gary ophylleae):— The first leaves produced 
by young seedlings rise up and close together at night. On a 
rather older seedling, two young leaves stood at noon at 55° 
above the horizon, and at night at 86°, so each had risen 31°. 
The angle, however, was less in some cases. Similar observations 
were occasionally made on young leaves (for the older ones 
moved very little) produced by nearly full-grown plants. Bata- 
lin says (' Flora,' Oct. 1st, 1873, p. 437) that the young leaves of 
Stellaria close up so completely at night that they form togetlier 
great buds. 

Sida (Malvaceae). — The nyctitropic movements of the leaves 
in this genus are remarkable in some respects. Batalin informs 
us (see also 'Flora,' Oct. 1st, 1873, p. 437) that those of .S'. 
napcea fall at night, but to what angle he cannot remember. 
The leaves of S. rhoiribi folia and return., on the other hand, rise 
up vertically, and are pressed against the stem. We have 
therefore here within the same genus, directly opposite move- 
ments. Again, the leaves of S. rhombifolia are furnished with 
a pulvinus, formed of a mass of small cells destitute of chloro- 
phyll, and with their longer axes perpendicular to the axis of 
the petiole. As measured along this latter line, these cells are 
only |-th of the length of those of the petiole ; but instead of 
being abruptly separated from them (as is usual with the pul- 
vinus in most plants), they graduate into the larger cells of the 
petiole. On the other hand, aS. napma^ according to Batalin, 
does not possess a pulvinus ; and he informs us that a gradation 
may be traced in the several species of the genus between these 
two states of the petiole. Sida rhom'bifolia presents another 
peculiarity, of which we have seen no other instance with leaves 



CriAP. VII. 



SLEEP OF LEAVES. 



325 



that sleep : for those on very young plants, though they rise 

somewhat in the evening, do not go to sleep, as we observed 

on several occasions ; whilst 

Fig. 126. 



those on rather older plants 
sleep in a conspicuous man- 
ner. For instance, a leaf 
(•85 of an inch in length) 
on a very young seedling 
2 inches high, stood at noon 
9° above the horizon, and 
at 10 P.M. at 28°, so it had 
risen only 19° ; another leaf 
(1*4 inch in length) on a 
seedling of the same height, 
stood at the same two pe- 
riods at 7° and 32°, and 
therefore had risen 25°. 
These leaves, which moved 
so little, had a fairly well- 
developed pulvinus. After 
an interval of some weeks, 
when the same seedlings 
were 2|- and 3 inches in 
height, some of the young 
leaves stood up at night 
quite vertically, and others 
were highly inclined; and 
so it was with bushes which 
were fully grown and were 
flowering. 

The movement of a leaf 
was traced from 9.15 a.m. 
on May 28th to 8.30 a.m. 
on the 30th. The tempera- 
ture was too low (15° — 16° 
C), and the illumination 
hardly sufficient ; conse- 
quently the leaves did not 
become quite so highly in- 



\10°S^.m.28^ 



6°4i5'a.m.30^\ 



8°30'a.m.S0^: 



\'9''d5'a.m.28^ 



Sida rhomhifolia : circumnutation and 
nyctitropic (or sleeps movements of 
aleaf on a yoiinst plant, 9^ inches 
high ; filament fixed to midrib of 
nearly full-grown leaf, 2§ inches in 
length ; movement traced under a 
skylight. Apex of leaf 5§ inches 
from the vertical glass, so diagram 
not greatly enlarged. 



326 MODIFIED CIRCUMNUTATION. Chap. VII. 

clined at night, as they had done previously and as they did 
subsequently in the hot -house; but the movements did not 
appear otherwise disturbed. On the first day the leaf sank till 
5.15 p.m.; it then rose rapidly and greatly till 10.5 p.m., and 
only a little higher during the rest of the night (Fig. 136). 
Early on the next day (29th) it fell in a slightly zigzag line 
rapidly until 9 a.m., by which time it had reached nearly the 
same place as on the previous morning. During the remainder 
of the day it fell slowly, and zigzagged laterally. The evening 
rise began after 4 p.m. in the same manner as before, and on 
the second morning it again fell rapidly. The ascending. and 
descending lines do not coincide, as may be seen in the diagram. 
On the 30th a new tracing was made (not here given) on a 
rather enlarged scale, as the apex of the leaf now stood 9 inches 
from the vertical glass. In order to observe more carefully the 
course pursued at the time when the diurnal fall changes into 
the nocturnal rise, dots were made every half-hour between 
4 P.M. and 10.30 p.m. This rendered the lateral zigzagging 
movement during the evening more conspicuous than in the 
diagram given, but it was of the same nature as there shown. 
The impression forced on our minds was that the leaf was 
expending superfluous movement, so that the great nocturnal 
rise might not occur at too early an hour. 

Abutilon Darioinii (Malvaceae). — The leaves on some very 
young plants stood almost horizontally during the day, and 
hung down vertically at night. Very fine plants kept in a 
large hall, lighted only from the roof, did not sleep at night, 
for in order to do so the leaves must be well illuminated during 
the day. The cotyledons do not sleep." Linngeus says that the 
leaves of his 8ida abutilon sink per])endicularly down at night, 
though the petioles rise. Prof. Pfeffer informs us that the 
leaves of a Malva, allied to M. syhestris, rise greatly at night; 
and this genus, as well as that of Hibiscus, are included by 
Linnseus in his list of sleeping plants. 

Anoda Wrightii (Malvaceae). — The leaves, produced by very 
young plants, when grown to a moderate size, sink at night 
either almost vertically down or to an angle of about 45° bener^h 
the horizon; for there is a considerable degree of variability in 
the amount of sinking at night, which depends in part on the 



Chap. VII. 



SLEEP OF LEAVES. 



327 



degree to which they have been illuminated during the day. 
But the leaves, whilst quite young, do not sink down at night, 
and this is a very unusual circumstance. The summit of the 
petiole, where it joins the blade, is developed into a pulvinus, 
and this is present in very young leaves which do not sleep ; 
though it is not so well defined as in older leaves. 

Gossypium (var. Nankin cotton, Malvacese). — Some young 
leaves, between 1 and 2 inches in length, borne by two seedlings 
6 and 7^ inches in height, stood horizontally, or were raised a 
little above the horizon at noon on July 8th and 9th ; but by 
10 P.M. they had sunk down to between 68° and 90° beneath 
the horizon. When the same plants had grown to double the 
above height, their leaves stood at night almost or quite verti- 
cally dependent. The leaves on some large plants of G. mariti- 
mum and Brazilense, which were kept in a very badly lighted 
hot-house, only occasionally sank much downwards at night, 
and hardly enough to be called sleep. 

Oxalis (Oxalidae). — In most of the species in this large genus 
the three leaflets sink vertically down at night; but as their 
sub -petioles are short 

the blades could not ^^°- ^'^^• 

assume this position 
from the want of space, 
unless they were in some 
manner rendered nar- 
rower; and this is ef- 
fected by their becom- 
ing more or less folded 
(Fig. 137). The angle 
formed by the two 
halves of the same leaf- 
let was found to vary 
in different individuals of several species between 92° and 150°; 
in three of the best folded leaflets of 0. fragrans it was 76°, 
74°, and 54°. The angle is often different in the three leaflets 
of the same leaf. As the leaflets sink down at night and become 
folded, their lower surfaces are brought near together (see B), 
or even into close contact ; and from this circumstance it might 
be thought that the object of the folding was the protection of 
22 





Oxalis acetosella : A, leaf seen from verti- 
cally above ; B, diagram of leaf asleep, 
also seen from vertically above. 



328 



MODIFIED CIRCUMNUTATION. Chap. VII. 



TdJ'a.m^S 



srs&jtm^h 




Itf'JjWm.^V 



Oxalis acetosella : circumnutation and 
nyctitropic movements of a nearly 
full-grown leaf, with filament at- 
tached to the midrib of one of the 
leaflets ; traced on vertical glass dur- 
ing 20 h, 45 m. 



their lower ^surfaces. If 
this had been the case, it 
would have formed a 
strongly marked exception 
to the rule, that when there 
is any difference in the de- 
gree of protection from ra- 
diation of the two surfaces 
of the leaves, it is always 
the upper surface which is 
the best protected. But 
that the folding of the leaf- 
lets, and consequent mu- 
tual approximation of their 
lower surfaces, serves mere- 
ly to allow them to sink 
down vertically, may be in- 
ferred from the fact that 
when the leaflets do not ra- 
diate from the summit of a 
common petiole, or, again, 
when there is plenty of 
room, from the sub-petioles 
not being very short, the 
leaflets sink down without 
becoming folded. This oc- 
curs with the leaflets of 
O. sensitiva, Plumierii and 
hupleurifoUa. 

There is no use in giv- 
ing a long list of the many 
species which sleep in the 
above .described manner. 
This holds good with spe- 
cies having rather fleshy 
leaves, like those of 0. car- 
nosa, or large leaves like 
those of 0. Ortegesii, or 
four leaflets like those of 



Chap. VII. SLEEP OF LEAVES. 329 

0. variabilis. There are, however, some species which show 
no signs of sleep, viz., 0. pentaphylla, enneaphylla, hirta, and 
rubella. We will now describe the nature of the movements 
in some of the species. 

Oxalis acetosella. — The movement of a leaflet, together with 
that of the main petiole, are shown in the following diagram 
(Fig. 138), traced between 11 a.m. on October 4th and 7.45 
A.M. on the 5th. After 5.30 p.m. on the 4th the leaflet sank 
rapidly, and at 7 p.m. depended vertically. For some time 
before it assumed this latter position, its movements could, of 
course, no longer be traced on the vertical glass, and the 
broken line in the diagram ought to be extended much further 
down in this and all other cases. By 6.45 a.m. on the follow- 
ing morning it had risen considerably, and continued to rise 
for the next hour; but, judging from other observations, it 
would soon have begun to fall again. Between 11 a.m. and 
6.30 p.m. the leaflet moved at least four times up and four times 
down before the great nocturnal fall commenced; it reached its 
highest point at noon. Similar observations were made on two 
other leaflets, with nearly the same results. Sachs and Pfeffer 
have also described briefly * the autonomous movements of the 
leaves of this plant. 

On another occasion the petiole of a leaf was secured to a 
little stick close beneath the leaflets, and a filament tipped with 
a bead of sealing-wax was afiixed 
to the midrib of one of them, and Fig. 129. 

a mark was placed close behind. 
At 7 P.M., when the leaflets were 
asleep, the filament depended ver- 
tically down, and the movements Oxalis acetosella : circumnuta- 
of the bead were then traced till Jion of leaflet when asleep; 

traced on vertical glass dur- 
10.40 P.M., as shown in the follow- ing 3 h. 40 m. 

ing diagram (Fig. 129) We here 

see that the leaflet moved a little from side to side, as well as a 

little up and down, whilst asleep. 

Oxalis Valdiviana. — The leaves resemble those of the last 




* Sachs in 'Flora,' 1863, p. 470, &c. ; Pfeffer, 'Die Period. Bewe- 
gungen,' &c., 1875, p. 53. 



330 MODIFIED CIRCUMNUTATION. Chap. VII. 

species, and the movements of two leaflets (the main petioles of 
both having been secured) were traced during two days ; but 
the tracings are not given, as they resembled that of 0. aceto- 
sella, with the exception that the up and down oscillations were 
not so frequent during the day, and there was more lateral 
movement, so that broader ellipses were described. The 
leaves awoke early in the morning, for by 6.45 a.m. on June 
12th and 13th they had not only risen to their full height, but 
had already begun to fall, that is, they were circumnutating. 
We have seen in the last chapter that the cotyledons, instead of 
sinking, rise up vertically at night. 

Oxalis Ortegesii. — The large leaves of this plant sleep like 
those of the previous species. The main petioles are long, and 
that of a young leaf rose 20° between noon and 10 p.m., whilst 
the petiole of an older leaf rose only 13°. Owing to this ris- 
ing of the petioles, and the vertical sinking of the large leaflets, 
the leaves become crowded together at night, and the whole 
plant then exposes a much smaller surface to radiation than 
during the day. 

Oxalis Plumierii. — In this species the three leaflets do not 
surround the summit of the petiole, but the terminal leaflet 
projects in the line of the petiole, with a lateral leaflet on each 
side. They all sleep by bending vertically downwards, but do 
not become at all folded. Tlie petiole is rather long, and, one 
having been secured to a stick, the movement of the terminal 
leaflet was traced during 25 h. on a vertical glass. It moved 
in a very simple manner, sinking rapidly after 5 p.m., and ris- 
ing rapidly early next morning. During the middle of the day 
it moved slowly and a little laterally. Consequently the ascend- 
ing and descending lines did not coincide, and a single great 
ellipse was formed each day. There was no other evidence of 
circumnutation, and this fact is of interest, as we shall here- 
after see. 

Oxalis sensitioa.— The leaflets, as in the last species, bend 
vertically down at niglit, without becoming folded. The much 
elongated main petiole rises considerably in the evening, but in 
some very young plants the rise did not commence until late at 
night. We have seen that the cotyledons, instead of sinking 
like the leaflets, rise up vertically at night. 



Chap. VII. 



SLEEP OF LEAVES. 



331 



Oxalis Impleurifolia. — This species is rendered remarkable 
by the petioles being foliaceous, like the pliyllodes of many 
Acacias. The leaflets are small, of a paler green and more ten- 
der consistence than the foliaceous petioles. The leaflet which 
was observed was '55 inch in 
length, and was borne by a petiole ^^^- -'^^^• 

3 inches long and "3 inches broad. 
It may be suspected that the 
leaflets are on the road to abor- 
tion or obliteration, as has actu- 
ally occurred with those of an- 
other Brazilian species, 0. rus- 
ciformis. Nevertheless, in the 
present species the nyctitropic 
movements are perfectly per- 
formed. The foliaceous petiole 
was first observed during 48 h., 
and found to be in continued cir- 
cumnutation, as shown in the ac- 
companying figure (Fig. 130). It 
rose durin'g the day and early part 
of the night, and fell during the 
remainder of the night and early 
morning ; but the movement was 
not sufficient to be called sleep. 
The ascending and descending 
lines did not coincide, so that an 
ellipse was formed each day. 
There was but little zigzagging; 
if the filament had been fixed 



longitudinally, we should prob- 



Oxalis hupleurifolia : circumnu- 
tation of foliaceous petiole, 
filament fixed obliquely 
across end of petiole ; move- 
ments traced on vertical 
glass from 9 a.m. June 26tli 
to 8.50 A.M. 28th. Apex of 
leaflet 4i inches from the 
glass, so movement not 
much magnified. Plant 9 
inches high, illuminated 
from above. Temp. 23|°- 
24r C. 



ably have seen that there was 
more lateral movement than ap- 
pears in the diagram. 

A terminal leaflet on another leaf was next observed (the 
petiole being secured), and its movements are shown in Fig. 
131. During the day the leaflets are extended horizontally, 
and at night depend vertically, and as the petiole rises during 
the day the leaflets have to bend down in the evening more than 



332 



MODIFIED CIRCUMNUTATIOX. Chap. YII. 



90°, so as to assume at night their vertical position. On the first 
day the leaflet simply moved up and down; on the second day 




it plainly circumnutated between 8 a.m. and 4.30 p.m. 
which hour the great evening fall commenced. 



after 



Chap. VII. SLEEP OF LEAVES. 333 

AverrTioa Mlinibi (Oxalidae). — It has long been known,* 
firstly, that the leaflets in this genus sleep; secondly, that 



Fig. 132. 




Averrhoa bilimhi : leaf asleep ; drawing reduced. 

they move spontaneously during the day; and thirdly, that 
they are sensitive to a touch ; but in none of these respects do 
they differ essentially from the species of Oxalis. They differ, 
however, as Mr, R, I. Lynch f has lately shown, in their spon- 
taneous movements being strongly marked. In the case of A. 
hilimbi, it is a wonderful spectacle to behold on a warm sunny 
day the leaflets one after the other sinking rapidly downwards, 
and again ascending slowly. Their movements rival those of 
Desmodium gyrans. At night the leaflets hang vertically down ; 
and now they are motionless, but this may be due to the oppo- 
site ones being pressed together (Fig. 132). The main petiole 
is in constant movement during the day, but no careful obser- 
vations were made on it. The following diagrams are graphic 
representations of the variations in the angle, which a given 
leaflet makes with the vertical. The observations were made 
as follows. The plant growing in a pot was kept in a high 
temperature, the petiole of the leaf to be observed pointing 
straight at the observer, being separated from him by a vertical 
pane of glass. The petiole was secured so that the basal joint, 
or pulvinus, of one of the lateral leaflets was at the centre of a 



* Dr. Bruce, Philosophical t 'Journal Linn. Soc.,' vol. xvi. 
Trans.,' 1785, p. 356. 1877, p. 231, 



334 



MODIFIED CmCUMNUTATION. Chap. VII. 



graduated are placed close behind the leaflet. A fine glass 
filament was fixed to the leaf, so as to project like a continua- 
tion of the midrib. This filament acted as an index; and as the 





leaf rose and fell, rotating about its basal joint, its angular 
movement could be recorded by reading off at short intervals 



Chap. VII. SLEEP OF LEAVES. 335 

of time the position of the glass filament on the graduated arc. 
In order to avoid errors of parallax, all readings were made by 
looking through a small ring painted on the vertical glass, in a 
line with the joint of the leaflet and the centre of the graduated 
arc. In the following diagrams the ordinates represent the 
angles which the leaflet made with the vertical at successive 
instants.* It follows that a fall in the curve represents an 
actual dropping of the leaf, and that the zero line represents a 
vertically dependent position. Fig. 133 represents the nature 
of the movements which occur in the evening, as soon as the 
leaflets begin to assume their nocturnal position. At 4.55 p.m. 
the leaflet formed an angle of 85° with the vertical, or was only 
5° below the horizontal ; but in order that the diagram might 
get into our page, the leaflet is represented falling from 75° 
instead of 85°. Shortly after 6 p.m. it hung vertically down, 
and had attained its nocturnal position. Between 6.10 and 
6.35 P.M. it performed a number of minute oscillations of about 
2° each, occupying periods of 4 or 5 m. The complete state of 
rest of the leaflet which ultimately followed is not shown in 
the diagram. It is manifest that each oscillation consists of a 
gradual rise, followed by a sudden fall. Each time the leaflet 
fell, it approached nearer to the nocturnal position than it did 
on the previous fall. The amplitude of the oscillations dimin- 
ished, while the periods of oscillation became shorter. 

In bright sunshine the leaflets assume a highly inclined 
dependent position. A leaflet in diffused light was observed 
rising for 25 m. A blind was then pulled up so that the plant 
was brightly illuminated (BR in Fig. 134), and within a minute 
it began to fall, and ultimately fell 47°, as shown in the dia- 
gram. This descent was performed by six descending steps, 
precisely similar to those by which the nocturnal fall is effected. 
The plant was then again shaded (SH), and a long slow rise 
occurred until another series of falls commenced at BR', when 
the sun was again admitted. In this experiment cool air was 



-•■ In all the diagrams 1 mm. in ment. In Figs. 133 and 134 the 

the horizontal direction represents temperature is represented (along 

one minute of time. Each mm. the ordinates) in the scale of 1 

in the vertical direction repre- mm. to each 0'1° C. In Fig. 135 

sents one degree of angular move- each mm. equals 0'2° F. 



336 



MODIFIED CIRCUMNUTATION. Chap. VII. 



allowed to enter by the windows being opened at the same time 
that the blinds were pulled up, so that in spite of the sun 
shining on the plant the temperature was not raised. 

The effect of an increase of temperature in diffused light is 
shown in Fig. 135. The temperature began to rise at 11.35 a.m. 




(in consequence of the fire being lighted), but by 12.43 a 
marked fall had occurred. It may be seen in the diagram that 
when the temperature was highest there were rapid oscillations 
of small amplitude, the mean position of the leaflet being at 
the time nearer the vertical. When the temperature began to 
fall, the oscillations became slower and larger, and the mean 
position of the leaf again approached the horizontal. The rate 



Chap. VII. 



SLEEP OF LEAVES. 



337 








1^3 

o a 



^ o 

t£bD 



t- o 



^ o 



338 



MODIFIED CIRCUMNUTATION. Chap. VII. 



Fig. 136. 



of oscillation was sometimes quicker than is represented in the 
above diagram. Thus, when the temperature was between 31° 
and 32° C, 14 oscillations of a few 
degrees occurred in 19 m. On the 
other hand, an oscillation may be 
much slower; thus a leaflet was ob- 
served (temperature 25° C.) to rise 
during 40 m. before it fell and com- 
pleted its oscillation, 

Porlieria Jiygrometrica (Zygo- 
phyllese). — The leaves of this plant 
(Chilian form) are from 1 to 1^ 
inches in length, and bear as many 
as 16 or 17 small leaflets on each 
side, which do not stand opposite 
one another. They are articulated 
to the petiole, and the petiole to the 
branch by a pulvinus. We must pre- 
mise that apparently two forms are 
confounded under the same name: 
the leaves on a bush from Chili, 
which was sent to us from Kew, bore 
many leaflets, whilst those on plants 
in the Botanic Garden at Wtirzburg 
bore only 8 or 9 pairs; and the whole 
character of the bushes appeared 
somewhat different. We shall also 
see that they differ in a remarkable 
physiological peculiarity. On the 
Chilian plant the petioles of the 
younger leaves on upright branches, 
stood horizontally during the day, 
and at night sank down vertically so 
as to depend parallel and close to 
the branch beneath. The petioles of 
rather older leaves did not become 
at night vertically depressed, but 
only highly inclined. In one instance we found a branch which 
had grown perpendicularly downwards, and the petioles on it 




Polieria hygrometrica : cir- 
cumnutation and nycti- 
tropic movemeuts of pet- 
iole of leaf, traced from 
9.35 A.M. July 7tli to 
about midnight on the 
8tli. Apex of leaf 7i 
inches from the vertical 
glass. Temp. 19h°-20h° C. 



Chap. VIT. SLEEP OF LEAVES. 339 

moved in the same direction relatively to the branch as just 
stated, and therefore moved upwards. On horizontal branches 
the younger petioles likewise move at night in the same direc- 
tion as before, that is, towards the branch, and are consequently 
then extended horizontally ; but it is remarkable that the older 
petioles on the same branch, though moving a little in the same 
direction, also bend downwards; they thus occupy a somewhat 
different position, relatively to the centre of the earth and to 
the branch, from that of the petioles on the upright branches. 
"With respect to the leaflets, they move at night towards the 
apex of the petiole until their midribs stand nearly parallel to it ; 
and they then lie neatly imbricated one over the other. Thus 
half of the upper surface of each leaflet is in close contact with 
half of the lower surface of the one next in advance ; and all the 
leaflets, excepting the basal ones, have the whole of their upper 
surfaces and half of their lower surfaces well protected. Those 
on the opposite sides of the same petiole do not come into close 
contact at night, as occurs with the leaflets of so many Legumi- 
nosae, but are separated by an open furrow ; nor could they exactly 
coincide, as they stand alternately with respect to one another. 

The circumnutation of the petiole of a leaf | of an inch in 
length, on an upright branch, was observed during 36 h., and 
is shown in the preceding diagram (Fig. 136). On the first 
morning, the leaf fell a little and then rose until 1 p.m., and 
this was probably due to its being now illuminated through a 
skylight from above; it then circumnutated on a very small 
scale round the same spot until about 4 p.m., when the great 
evening fall commenced. During the latter part of the night 
or very early on the next morning the leaf rose again. On the 
second day it fell during the morning till 1 p.m., and this no 
doubt is its normal habit. From 1 to 4 p.m it rose in a zigzag 
line, and soon afterwards the great evening fall commenced. 
It thus completed a double oscillation during the 24 h. 

The specific name given to this plant by Ruiz and Pavon, 
indicates that in its native arid home it is affected in some man- 
ner by the dryness or dampness of the atmosphere.* In the 



*'Systeina Veg. Florae Peru- 1798. We cannot understand the 
vianse et Chilensis,' torn. i. p. 95, account given by the authors of 



340 MODIFIED CIRCUMNUTATION. Chap. VIT. 

Botanic Garden at Wtirzburg, there was a plant in a pot out of 
doors which was daily watered, and another in the open ground 
wtiich was never watered. After some hot and dry weather 
there was a great difference in the state of the leaflets on these 
two plants; those on the unwatered plant in the open ground 
remaining half, or even quite, closed during the day. But 
twigs cut from this bush, with their ends standing in water, or 
wholly immersed in it, or kept in damp air under a bell-glass, 
opened their leaves though exposed to a blazing sun; whilst 
those on the plant in the ground remained closed. The leaves 
on this same plant, after some heavy rain, remained open for 
two days; they then became half closed during two days; and 
after an additional day were quite closed. This plant was 
now copiously watered, and on the following morning the 
leaflets were fully expanded. The other plant growing in a 
pot, after having been exposed to heavy rain, was placed before 
a window in the Laboratory, with its leaflets open, and they 
remained so during the daytime for 48 h. ; but after an addi- 
tional day were half closed. The plant was then watered, and 
the leaflets on the two following days remained open. On the 
third day they were again half closed, but on being again wa- 
tered remained open during the two next days. From these 
several facts we may conclude that the plant soon feels the 
want of water; and that as soon as this occurs, it partially or 
quite closes its leaflets, which in their then imbricated condition 
expose a small surface to evaporation. It is therefore probable 
that this sleep -like movement, which occurs only when the 
ground is dry, is an adaptation against the loss of moisture. 

A bush about 4 feet in height, a native of Chili, which was 
thickly covered with leaves, behaved very differently, for dur- 
ing the day it never closed its leaflets. On July 6th the earth 
in the small pot in which it grew appeared extremely dry, and 
it was given a very little water. After 21 and 22 days (on the 
37th and 28th), during the whole of which time the plant did 
not receive a drop of water, the leaves began to droop, but they 



the behaviour of this plant in its appears as if the brightness of 
native home. There is much the sky largely determined the 
about its power of foretelling opening and closing of the leaf- 
changes in the weather ; and it lets. 



Char VII. SLEEP OF LEAVES. 341 

showed no signs of closing during the day. It appeared almost 
incredible that any plant, except a fleshy one, could have kept 
alive in soil so dry, which resembled the dust on a road. On 
the 29th, when the bush was shaken, some leaves fell off, and 
the remaining ones were unable to sleep at night. It was 
therefore moderately watered, as well as syringed, late in the 
evening. On the next morning (30th) the bush looked as fresh 
as ever, and at night the leaves went to sleep. It may be added 
that a small branch while growing on the bush was enclosed, 
by means of a curtain of bladder, during 13 days in a large 
bottle half full of quicklime, so that the air within must have 
been intensely dry ; yet the leaves on this branch did not suffer 
in the least, and did not close at all during the hottest days. 
Another trial was made with the same bush on August 2nd 
and 6th (the soil appearing at this latter date extremely dry), 
for it was exposed out of doors during the whole day to 
the wind, but the leaflets showed no signs of closing. The 
Chilian form therefore differs widely from the one at "Wtirz- 
burg, in not closing its leaflets when not suffering from the 
want of water; and it can live for a surprisingly long time 
without water. 

Tropceolnm majus (?) (cultivated var.) (Tropseolese). — Several 
plants in pots stood in the greenhouse, and the blades of tlie 
leaves which faced the front-lights were during the day highly 
inclined and at night vertical ; whilst the leaves on the back of 
the pots, though of course illuminated through the roof, did 
not become vertical at night. We thought, at first, that this 
difference in their positions was in some manner due to heli- 
otropism, for the leaves are highly heliotropic. The true expla- 
nation, however, is that unless they are y\ ell illuminated dur- 
ing at least a part of the day they do not sleep at night ; and a 
little difference in the degree of illumination determines whether 
or not they shall become vertical at night. We have observed 
no other so well-marked a case as this, of the irfluence of pre- 
vious illumination on nyctitropic movements. The leaves pre- 
sent also another peculiarity in their habit of rising or awaking 
in the morning, being more strongly fixed or inherited than 
that of sinking or sleeping at night. The movements are caused 
by the bending of an upper part of the petiole, between \ and 



342 MODIFIED CIRCUMNUTATION. Chap. VII. 

1 inch in length ; but the part close to the blade, for about J of 
an inch in length, does not bend and always remains at right 
angles to the blade. The bending portion does not present any 
external or internal difference in structure from the rest of the 
petiole. We will now give the experiments on which the above 
conclusions are founded. 

A large pot with several plants was brought on the morning 
of Sept. 3rd out of the greenhouse and placed before a north- 
east window, in the same position as before with respect to tlie 
light, as far as that was possible. On the front of the plants, 
24 leaves were marked with thread, some of which had their 
blades horizontal, but the greater number were inclined at 
about 45°, beneath the horizon; at night all these, without ex- 
ception, became vertical. Early on the following morning 
(4th) the^ reassumed their former positions, and at night again 
became vertical. On the 5th the shutters were opened at 6.15 
A.M., and by 8.18 a.m., after the leaves had been illuminated for 

2 h. 3 m., and had acquired their diurnal position, they were 
placed in a dark cupboard. They were looked at twice during 
the day and thrice in the evening, the last time at 10.30 p.m., 
and not one had become vertical. At 8 a.m. on the following 
morning (6thj they still retained the same diurnal position, and 
were now replaced before the north-east window. At night all 
the leaves which had faced the light had their petioles curved 
and their blades vertical; whereas none of the leaves on the 
back of the plants, although they had been moderately illumi- 
nated by the diffused light of the room, were vertical. They 
were now at night placed in the same dark cupboard; at 9 a.m. 
on the next morning (7th) all those which had been asleep had 
reassumed their diurnal position. The pot was then placed for 

3 h. in the sunshine, so as to stimulate the plants; at noon they 
were placed before the same north-east window, and at night 
the leaves slept in the usual manner and awoke on the follow- 
ing morning. At noon on this day (8th) the plants, after hav- 
ing been left before the north east window for 5 h. 45 m. and 
thus illuminated (though not brightly, as the sky was cloudy 
during the whole time), were replaced in the dark cupboard, 
and at 3 p.m. the position of the leaves was very little, if at all, 
altered, so that they are not quickly affected by darkness; but 



Chap. VII. SLEEP OF LEAVES. 343 

by 10.15 P.M. all the leaves which had faced the north-east sky 
during the 5 h. 45 m. of illumination stood vertical, whereas 
those on the back of the plant retained their diurnal position. On 
the following morning (9th) the leaves awoke as on the two for- 
mer occasions in the dark, and they were kept in the dark dur- 
ing the whole day ; at night a very few of them became vertical, 
and this was the one instance in which we observed any inher- 
ited tendency or habit in this plant to sleep at the proper time. 
That it was real sleep was shown by these same leaves reassum- 
ing their diurnal position on the following morning (10th) 
whilst still kept in the dark. 

The pot was then (9.45 a.m. 10th) replaced, after having 
been kept for 36 h. in darkness, before the north-east window ; 
and at night the blades of all the leaves (excepting a few on 
the back of the plants) became conspicuously vertical. 

At 6.45 A.M. (11th) after the plants had been illuminated on 
the same side as before during only 25 m., the pot was turned 
round, so that the leaves which had faced the light now faced 
the interior of the room, and not one of these went to sleep at 
night ; whilst some, but not many, of those which had formerly 
stood facing the back of the room and which had never before 
been well illuminated or gone to sleep, now assumed a vertical 
position at night. On the next day (13th) the plant was turned 
round into its original position, so that the same leaves faced 
the light as formerly, and these now went to sleep in the usual 
manner. We will only add that with some young seedlings 
kept in the greenhouse, the blades of the first pair of true leaves 
(the cotyledons being hypogean) stood during the day almost 
horizontally and at night almost vertically. 

A few observations were subsequently made on the circum- 
nutation of three leaves, whilst facing a north-east window; 
but the tracings are not given, as the leaves moved somewhat 
towards the light. It was, however, manifest that they rose 
and fell more than once during the daytime, the ascending and 
descending lines being in parts extremely zigzag. The noc- 
turnal fall commenced about 7 p.m., and the leaves had risen 
considerably by 6.45 a.m. on the following morning. 

LeguminoscB.— This Family includes many more genera with 
sleeping species than all the other families put together. The 
23 



344 MODIFIED CIRCUMNUTATI02T. Chap. VII. 

number of the tribes to which each genus belongs, according to 
Bentham and Hooker's arrangement, has been added. 

Crotolaria (sp. ?) (Tribe 2). — This plant is monophyllous, 
and we are informed by Mr. T. Thiselton Dyer that the leaves 
rise up vertically at night and press against the stem. 

Lupinus (Tribe 2). — The palmate or digitate leaves of the 
species in this large genus sleep in three different manners. 
One of the simplest, is that all the leaflets become steeply 
inclined downwards at night, having been during the day 




A B 

Lupinus pilosus : A, leaf seen from vertically above in daytime ; B, leaf 
asleep, seen laterally at night. 

extended horizontally. This is shown in the accompanying 
figures (Fig. 137), of a leaf of L. pilosus, as seen during the 
day from vertically above, and of anotker leaf asleep with the 
leaflets inclined downwards. As in this position they are 
crowded together, and as they do not become folded like those 
in the genus Oxalis, they cannot occupy a vertically dependent 
position ; but they are often inclined at an angle of 50° beneath 
the horizon. In this species, whilst the leaflets are sinking, 
the petioles rise up, in two instances when the angles were 
measured to the extent of 23°. The leaflets of Z. sub-cai^osus 
and arhoreus, which were horizontal during the day, sank down 
at night in nearly the same manner; the former to an angle of 
38°, and the latter of 36°, beneath the horizon ; but their peti- 
oles did not move in any plainly perceptible degree. It is, 
however, quite possible, as we shall presently see, that if a large 
number of plants of the three foregoing and of the following 
species were to be observed at all seasons, some of the leaves 
would be found to sleep in a different manner. 



Chap. VII. SLEEP OF LEAVES. 345 

In the two following species the leaflets, instead of moving 
downwards, rise at night. With L. Hartwegii some stood at 
noon at a mean angle of 36° above the horizon, and at night 
at 51°, thus forming together a hollow cone with moderately 
steep sides. The petiole of one leaf rose 14° and of a second 
11° at night. With L, luteus a leaflet rose from 47° at noon to 
65° above the horizon at night, and another on a distinct leaf 
rose from 45° to 69°. The petioles, however, sink at night to 
a small extent, viz., in three instances by 2°, 6°, and 9° 30'. 
Owing to this movement of the petioles, the outer and longer 
leaflets have to bend up a little more than the shorter and inner 
ones, in order that all should stand symmetrically at night. 
We shall presently see that some leaves on the same individual 
plants of L. luteus sleep in a very different manner. 

We now come to a remarkable position of the leaves when 
asleep, which is common to several species of Lupines. On the 
same leaf the shorter leaflets, which generally face the centre 
of the plant, sink at night, whilst the longer ones on the oppo- 
site side rise ; the intermediate and lateral ones merely twisting 
on their own axes. But there is some variability with respect 
to which leaflets rise or fall. As might have been expected 
from such diverse and complicated movements, the base of each 
leaflet is developed (at least in the case of L. luteus) into a pul- 
vinus. The result is that all the leaflets on the same leaf stand 
at night more or less highly inclined, or even quite vertically, 
forming in this latter case a vertical star. This occurs with the 
leaves of a species purchased under the name of X. pubescens; 
and in the accompanying figures we see at A (Fig. 138) the 
leaves in their diurnal position; and at B the same pldnt at 
night with the two upper leaves having their leaflets almost 
vertical. At C another leaf, viewed laterally, is shown with 
the leaflets quite vertical. It is chiefly or exclusively the 
youngest leaves which form at night vertical stars. But there 
is much variability in the position of the leaves at night on the 
same plant ; some remaining with their leaflets almost horizon- 
tal, others forming more or less highly inclined or vertical stars, 
and some with all their leaflets sloping downwards, as in our 
first class of cases. It is also a remarkable fact, that although 
all the plants produced from the same lot of seeds were identi- 



346 



MODIFIED CIRCUMNUTATION. Chap. VII. 



cal in appearance, yet some individuals at night had the leaflets 
of all their leaves arranged so as to form more or less highly 
inclined stars; others had them all sloping downwards and 
never forming a star; and others, again, retained them either 
in a horizontal position or raised them a little. 

Fig. 138. 




Lupimi^ piiheseens : A, leaf viewed laterally during the day ; B, same 
leaf at night ; C. another leaf with the leaflet forming a vertical 
star at night. Figures reduced. 



"We have as yet referred only to the different positions of the 
leaflets of L. pubescens at night ; but the petioles likewise differ 
in their movements. That of a young leaf which formed a 
highly inclined star at night, stood at noon at 42° above the 
horizon, and during the night at 72°, so had risen 30°. The 



Chap. VII. SLEEP OF LEAVES. 347 

petiole of another leaf, the leaflets of which occupied a similar 
position at night, rose only 6°. On the other hand, the petiole 
of a leaf with all its leaflets sloping down at night, fell at this 
time 4°. The petioles of two rather older leaves were subse- 
quently observed ; both of which stood during the day at exactly 
the same angle, viz., 50° above the horizon, and one of these rose 
7°— 8°, and the other fell 3°— 4° at night. 

We meet with cases like that of L. pubescens with some other 
species. On a single plant of L. mutabilis some leaves, w^hich 
stood horizontally during the day, formed highly inclined stars 
at night, and the petiole of one rose 7°. Other leaves which 
likewise stood horizontally during the day, had at night all 
their leaflets sloping downwards at 46° beneath the horizon, but 
their petioles had hardly moved. Again, L. luteus offered a still 
more remarkable case, for on two leaves, the leaflets which stood 
at noon at about 45° above the horizon, rose at night to 65° and 
69°, so that they formed a hollow cone with steep sides. Four 
leaves on the same plant, which had their leaflets horizontal at 
noon, formed vertical stars at night; and three other leaves 
equally horizontal at noon, had all their leaflets sloping down- 
wards at night. So that the leaves on this one plant assumed 
at night three different positions. Though we cannot account 
for this fact, we can see that such a stock might readily give 
birth to species having widely different nyctitropic habits. 

Little more need be said about the sleep of the species of Lu- 
pinus ; several, namely, L. polypJiyllus, nanus, Menziesii, speciosus, 
and albifrons, though observed out of doors and in the green- 
house, did not change the position of their leaves sufliciently at 
night to be said to sleep. From observations made on two 
sleeping species, it appears that, as with Tropoeolum majus, the 
leaves must be well illuminated during the day in order to sleep 
at night. For several plants kept all day in a sitting-room 
with north-east windows, did not sleep at night; but when the 
pots were placed on the following day out of doors, and were 
brought in at night, they slept in the usual manner. The trial 
was repeated on the following day and night with the same 
result. 

Some observations were made on the circumnutation of the 
leaves of L. luteus and arhoreus. It will suffice to say that the 



3^8 



MODIFIED CmCUMNUTATION. Chap. VII. 



leaflets of the latter exhibited a double oscillation in the course 
of 24 h. ; for they fell from the early morning until 10.15 a.m., 
then rose and zigzagged greatly till 4 p.m., after which hour the 
great nocturnal fall commenced. By 8 a.m. on the following 
morning the leaflets had risen to their proper height. We have 
seen in the fourth chapter, that the leaves of Lupinus speciosus, 
which do not sleep, circumnutate to an extraordinary extent, 
making many ellipses in the course of the day. 

Gytisus (Tribe 2), Trigonella and Medicago (Tribe 3).— Only 
a few observations were made on these three genera. The 
petidles on a young plant, about a foot in height, of Gytisus 
fragrans rose at night, on one occasion 23 and° on another 33°. 
The three leaflets also bend upwards, and at the same time 
approach each other, so that the base of the central leaflet 
overlaps the bases of the two lateral leaflets. They bend 
up so much that they press against the stem ; and on looking 
down on one of these young plants from vertically above, the 
lower surfaces of the leaflets are visible; and thus their upper 
surfaces, in accordance with the general rule, are best protected 
frofd radiation. Whilst the leaves on these young plants were 
thus behaving, those on an old bush in full flower did not sleep 
at night. 

Fig. 139. 





A B 

Medicago marina : A, leaves during the day ; B, leaves asleep at night. 

Trigonella Gretica resembles a Melilotus in its sleep, which 
will be immediately described. According to M. Royer,* the 



Annales des Sc. Nat. Bot.' (5th series), ix. 1868, p. 368. 



Chap. VII. 



SLEEP OF LEAVES. 



149 



leaves of Medicago maculata rise up at night, and ' ' se tenversent 
un peu de maniere a presenter obliquement au ciel leur face 
inferieure." A drawing is here given (Fig. 139) of the leaves of 
M. marina awake and asleep ; and this would almost serve for 
Cytims fragrans in the same two states. 



Fig. 140 




Melilotus officinalis : A, leaf during the daytime. B, another leaf asleep. 
C, a leaf asleep as viewed from vertically above ; but in this case 
the terminal leaflet did not happen to be in such close contact with 
the lateral one, as is usual. 



Melilotus (Tribe 3). — The species in this genus sleep in a 
remarkable manner. The three leaflets of each leaf twist through 
an angle of 90°, so that their blades stand vertically at night 
with one lateral edge presented to the zenith (Fig. 140). We 
shall best understand the other and more complicated move- 
ments, if we imagine ourselves always to hold the leaf with the 
tip of the terminal leaflet pointed to the north. The leaflets in 
becoming vertical at night could of course twist so that their 
upper surfaces should face to either side ; but the two lateral 



350 MODIFIED CIRCUMNUTATION. Chap. VII. 

leaflets always twist so that this surface tends to face the north, 
but as they move at the same time towards the terminal leaflet, 
the upper surface of the one faces about N.N.W., and that of 
the other N.N.E. The terminal leaflet behaves differently, for 
it twists to either side, the upper surface facing sometimes east 
and sometimes west, but rather more commonly west than east. 
The terminal leaflet also moves in another and more remarkable 
manner, for whilst its blade is twisting and becoming vertical, 
the whole leaflet bends to one side, and invariably to the side 
towards which the upper surface is directed; so that if this 
surface faces the west the whole leaflet bends to the west, until 
it comes into contact with the upper and vertical surface of 
the western lateral leaflet. Tims the upper surface of the 
terminal and of one of the two lateral leaflets is well protected. 

The fact of the terminal leaflet twisting indifferently to either 
side and afterwards bending to the same side, seemed to us so 
remarkable, that we endeavoured to discover the cause. We 
imagined that at the commencement of the movement it might 
be determined by one of the two halves of the leaflet being 
a little heavier than the other. Therefore bits of wood were 
gummed on one side of several leaflets, but this produced no 
effect; and they continued to twist in the same direction as 
they had previously done. In order to discover whether the 
same leaflet twisted permanently in the same direction, black 
threads were tied to 20 leaves, the terminal leaflets of which 
twisted so that their upper surfaces faced west, and 14 white 
threads to leaflets which twisted to the east. These were ob- 
served occasionally during 14 days, and they all continued, with 
a single exception, to twist and bend in the same direction ; for 
one leaflet which had originally faced east, was observed after 
days to face west. The seat of both the twisting and bending 
movement is in the pulvinus of the sub-petioles. 

We believe that the leaflets, especially the two lateral ones, 
in performing the above described complicated movements gen- 
erally bend a little downwards ; but we are not sure of this, for, 
as far as the main petiole is concerned, its nocturnal movement 
is largely determined by the position which the leaf happens to 
occupy during the day. Thus one main petiole was observed 
to rise at night 59°, whilst three others rose only 7° and 9°. 



Chap. VII. SLEEP OF LEAVES. 351 

The petioles and sub-petioles are continually circumnutating 
during the whole 24 h., as we shall presently see. 

The leaves of the following 15 species, M. officinalis, suaveo- 
lens, parvijlora, alba, infesta, dentata, gracilis, sulcata, elegans, 
cc&rulea, petitpierreana,, macrorrhiza, Italica, secundijlora, and 
Taurica, sleep in nearly the same manner as just described ; but 
the bending to one side of the terminal leaflet is apt to fail un- 
less the plants are growing vigorously. With M. petitpierreana 
and secundijlora the terminal leaflet was rarely seen to bend to 
one side. In young plants of M. Italica it bent in the usual 
manner, but with old plants in full flower, growing in the 
same pot and observed at the same hour, viz., 8.30 p.m. none of 
the terminal leaflets on several scores of leaves had bent to one 
side, though they stood vertically, nor had the two lateral 
leaflets, though standing vertically, moved towards the termi- 
nal one. At 10.30 p.m. and again one hour after midnight, the 
terminal leaflets had become very slightly bent to one side, and 
the lateral leaflets had moved a very little towards the terminal 
one, so that the position of the leaflets even at this late hour 
was far from the ordinary one. Again, with M. Taurica the 
terminal leaflets were never seen to bend towards either of the 
two lateral leaflets, though these, whilst becoming vertical, 
had bent towards the terminal one. The sub -petiole of the 
terminal leaflet in this species is of unusual length, and if the 
leaflet had bent to one side, its upper surface could have come 
into contact only with the apex of either lateral leaflet ; 
and this, perhaps, is the meaning of the loss of the lateral 
movement. 

The cotyledons do not sleep at night. The first leaf con- 
sists of a single orbicular leaflet, which twists at night so that 
the blade stands vertically. It is a remarkable fact that with 
M. Taurica, and in a somewhat less degree with M. macrorrhiza 
Q.ndi petitpierreana, all the many small and young leaves produced 
during the early spring from shoots on some cut-down plants 
in the greenhouse, slept in a totally different manner from the 
normal one; for the three leaflets, instead of twisting on their 
own axes so as to present their lateral edges to the zenith, 
turned upwards and stood vertically with their apices pointing 
to the zenith. They thus assumed nearly the same ])osition as 



352 MODIFIED CIRCUMNUTATION. Chap. VII. 

in the allied genus Trifolium ; and on the same principle that 
embryological characters reveal the lines of descent in the ani- 
mal kingdom, so the movements of the small leaves in the above 
three species of Melilotus, perhaps indicate that this genus is 
descended from a form which was closely allied to and slept 
like a Trifolium. Moreover, there is one species, M. messanensis, 
the leaves of which, on full-grown plants between 2 and 3 feet 
in height, sleep like the foregoing small leaves and like those 
of a Trifolium. We were so much surprised at this latter case 
that, until the flowers and fruit were examined, we thought 
that the seeds of some Trifolium had been sown by mistake 
instead of those of a Melilotus. It appears therefore probable 
that M. messanensis has either retained or recovered a primor- 
dial habit. 

The circumnutation of a leaf of M. officinalis was traced, the 
stem being left free ; and the apex of the terminal leaflet de- 
scribed three laterally extended ellipses, between 8 a.m. and 
4 P.M. ; after the latter hour the nocturnal twisting movement 
commenced. It was afterwards ascertained that the above 
movement was compounded of the circumnutation of the stem 
on a small scale, of the main petiole which moved most, and of 
the sub-petiole of the terminal leaflet. The main petiole of a 
leaf having been secured to a stick, close to the base of the sub- 
petiole of the terminal leaflet, the latter described two small 
ellipses between 10.30 a.m. and 3 p.m. At 7.15 p.m., after this 
same leaflet (as well as another) had twisted themselves into 
their vertical nocturnal position, they began to rise slowly, and 
continued to do so until 10.35 p.m., after which hour they were 
no longer observed. 

As M. messanensis sleeps in an anomalous manner, unlike 
that of any other species in the genus, the circumnutation of a 
terminal leaflet, with the stem secured, was traced during two 
days. On each morning the leaflet fell, until about noon, and 
then began to rise very slowly ; but on the first day the rising 
movement was interrupted between 1 and 3 p.m. by the forma- 
tion of a laterally extended ellipse, and on the second day, at 
the same time, by two smaller ellipses. The rising movement 
then recommenced, and became rapid late in the evening, when 
the leaflet was beginning to go to sleep. The awaking or sink- 



Chap. VII. SLEEP OF LEAVES. 353 

ing movement had already commenced by 6.45 a.m. on both 
mornings. 

TrifoUum (Tribe 3). — The nyctitropic movements of 11 
species were observed, and were found to be closely similar. 
If we select a leaf of T. repens having an upright petiole, and 
with the three leaflets expanded horizontally, the two lateral 
leaflets will be seen in the evening to twist and approach each 
other, until their upper surfaces come into contact. At the 



141. 





A B 

TrifoUum repens : A, leaf during the day ; B, leaf asleep at night. 

same time they bend downwards in a plane at right angles to 
that of their former position, until their midribs form an angle 
of about 45° with the upper part of the petiole. This peculiar 
change of position requires a considerable amount of torsion in 
the pulvinus. The terminal leaflet merely rises up without any 
twisting, and bends over until it rests on and forms a roof 
over the edges of the now vertical and united lateral leaflets. 
Thus the terminal leaflet always passes through an angle of at 
least 90°, generally of 130° or 140°, and not rarely— as was often 
observed with T. subterraneum—oi 180°. In this latter case 
the terminal leaflet stands at night horizontally (as in Fig. 141), 
witli its lower surface fully exposed to the zenith. Besides 
the difference in the angles, at which the terminal leaflets stand 
at night in the individuals of the same species, the degree to 
which, the lateral leaflets approach each other often likewise 
differs. 

We have seen that the cotyledons of some species and not 
of others rise up vertically at night. The first true leaf is gen- 
erally unifoliate and orbicular; it always rises, and either stands 
vertically at night or more commonly bends a little over so as 
to expose the lower surface obliquely to the zenith, in the same 
manner as does the terminal leaflet of the mature leaf. But it 



354 MODIFIED CmCUMNUTATION. Chap. VII. 

does not twist itself like the corresponding first simple leaf of 
Melilotus. With T. Pannonicum the first true leaf was gener- 
ally unifoliate, but sometimes trifoliate, or again partially lobed 
and in an intermediate condition. 

Circumnutation. — Sachs described in 1863* the spontaneous 
up and down movements of the leaflets of T. incarnatum, when 
kept in darkness. Pfeffer made many observations on the simi- 
lar movements in T. pratense.i He states that the terminal 
leaflet of this species, observed at different times, passed 
through angles of from 30° to 120° in the course of from 1^ to 
4 h. We observed the movements of T. subterraneum, resupi- 
natum, and repens. 

Trifolium subterraneum. — A petiole was secured close to the 
base of the three leaflets, and the movement of the terminal 
leaflet was traced during 26|^ h., as shown in the figure on the 
next page. 

Between 6.45 a.m and 6 p.m. the apex moved 3 times up 
and 3 times down, completing 3 ellipses in 11 h. 15 m. The 
ascending and descending lines stand nearer to one another 
than is usual with most plants, yet there was some lateral mo- 
tion. At 6 P.M. the great nocturnal rise commenced, and on 
the next morning the sinking of the leaflet was continued until 
8.30 A.M., after which hour it circumnutated in the manner just 
described. In the figure the great nocturnal rise and the morn- 
ing fall are greatly abbreviated, from the want of space, and 
are merely represented by a short curved line. Tlie leaflet 
stood horizontally when at a point a little beneath the middle 
of the diagram ; so that during the daytime it oscillated almost 
equally above and beneath a horizontal position. At 8.30 a.m. 
it stood 48° beneath the horizon, and by 11.30 a.m. it had risen 
50° above the horizon ; so that it passed through 98° in 3 h. 
By the aid of the tracing we ascertained that the distance trav- 
elled in the 3 h. by the apex of this leaflet was 1*03 inch. If 
we look at the figure, and prolong upwards in our mind's eye 
the short curved broken line, which represents the nocturnal 
course, we see that the latter movement is merely an exaggera- 



* ' Flora,' 1863, p. 497. 

t ' Die Period. Bewegungeii, 1875, pp. 35, 52. 



Chap. VII. 



SLEEP OF LEAVES. 



355 



tion or prolongation of 
one of the diurnal el- 
lipses. The same leaflet 
had been observed on 
the previous day, and 
the course then pursued 
was almost identically 
the same as that here 
described. 

Trifolinm resupina- 
tum. — A plant left en- 
tirely free was placed 
before a north-east win- 
dow, in such a position 
that a terminal leaflet 
projected at right angles 
to the source of the 
light, the sky being uni- 
formly clouded all day. 
The movements of this 
leaflet were traced dur- 
ing two days, and on 
both were closely simi- 
lar. Those executed on 
the second day are shown 
in Fig. 143. The obli- 
quity of the several lines 
is due partly to the man- 
ner in which the leaflet 
was viewed, and partly 
to its having moved a 
little towards the light. 
From 7.50 a.m. to 8.40 
A.M. the leaflet fell, that 
is, the awakening move- 
ment was continued. It 
then rose and moved a 
little laterally towards 
the light. At 12.30 it 




356 



MODIFIED CmCUMNUTATION. Chap. VII. 



Fisr. 143. 



retrograded, and at 2.30 resumed its original course, having 
thus completed a small ellipse during the middle of the day. 
In the evening it rose rapidly, and by 8 a.m. on the following 
morning had returned to exactly the same spot as on the pre- 
vious morning. The line representing the nocturnal course 
ought to be extended much higher up, and is here abbreviated 
into a short, curved, broken line. The terminal leaflet, there- 
fore, of this species de- 
scribed during the day- 
time only a single addi- 
tional ellipse, instead of 
two additional ones, as in 
the case of T. suhterra- 
neum. But we should re- 
member that it was shown 
in the fourth chapter that 
the stem circumnutates, as 
no doubt does the main 
petiole and the sub-peti- 
oles; so that the move- 
ment represented in Fig. 
143 is a compounded one. 
We tried to observe the 
movements of a leaf kept 
during the day in darkness, but it began to go to sleep after 
2 h. 15 m., and this was well pronounced after 4 h. 30 m. 

TrifoUum repens. — A stem was secured close to the base of 
a moderately old leaf, and the movement of the terminal leaflet 
was observed during two days. This case is interesting solely 
from the simplicity of the movements, in contrast with those of 
the two preceding species. On the first day the leaflet fell 
between 8 a.m. and 3 p.m., and on the second between 7 a.m. 
and 1 P.M. On both days the descending course was somewhat 
zigzag, and this evidently represents the circumnutating move- 
ment of the two previous species during the middle of the day. 
After 1 P.M., Oct. 1st (Fig. 144), the leaflet began to rise, but 
the movement was slow on both days, both before and after 
this hour, until 4 p.m. The rapid evening and nocturnal rise 
then commenced. Thus in this species the course during 24 h. 




TrifoUum resupinatum : circurtinutation 
and nyctitropic movements of the 
terminal leaflet during 24 hours. 



Chap. VII. 



SLEEP OF LEAVES. 



357 



Fig. 144. 



consists of a single great ellipse; in T. resupinatum of two 
ellipses, one of which includes the nocturnal movement and is 
much elongated; and in T. subterra- 
r.eum of three ellipses, of which the 
nocturnal one is likewise of great 
length. 

Securigera coronilla (Tribe 4). — The 
leaflets, which stand opposite one an- 
other and are numerous, rise up at 
night, come into close contact, and 
bend backwards at a moderate angle 
towards the base of the petiole. 

Lotus (Tribe 4). — The nyctitropic 
movements of 10 species in this genus 
were observed, and found to be the 
same. The main petiole rises a little 
at night, and the three leaflets rise till 
they become vertical, and at the same 
time approach each other. This 
was conspicuous with L. Jacohceus, in 
which the leaflets are almost linear. 
In most of the species the leaflets rise 
so much as to press against the stem, 
and not rarely they become inclined a 
little inwards with their lower surfaces 
exposed obliquely to the zenith. This 
was clearly the case with L. major^ as 
its petioles are unusually long, and the 
leaflets are thus enabled to bend fur- 
ther inwards. The young leaves on 
the summits of the stems close up at 
night so much, as often to resemble 
large buds. Tlie stipule-like leaflets, which are often of large 
size, rise up like the other leaflets, and press against the stem 
(Fig. 145). All the leaflets of L. Gebelii^ and probably of the 
other species, are provided at their bases with distinct pulvini, 
of a yellowish colour, and formed of very small cells. The cir- 
cumnutation of a terminal leaflet of L. perigrinus (with the stem 
secured) was traced during two days, but the movement was so 




Trifolmm repens : circum- 
nutation and nyctitropic 
movements of a near- 
ly full - grown terminal 
leaflet, traced on a ver- 
tical glass from 7 A.M. 
Sept, 30th to 8 A.M. Oct. 
1st. Nocturnal course, 
represented by curved 
broken line, much ab- 
breviated. 



358 



MODIFIED CIRCUMNUTATION. Chap. VII. 



simple that it is not worth while to give the diagram. The leaf- 
let fell slowly from the early morning till about 1 p.m. It then 
rose gradually at first, but rapidly late in the evening. It occa- 



Fig. 145. 




A B 

A, stem with leaves awake during the day ; 
SS, stipule-like leaflets. 



Lotus Creticns 

leaves asleep at night. 



B, with 



sionally stood still for about 20 m. during the day, and some- 
times zigzagged a little. The movement of one of the basal, 
stipule-like leaflets was likewise traced in the same manner and 
at the same time, and its course was closely similar to that 
of the terminal leaflet. 

In Tribe 5 of Bentham and Hooker, the sleep-movements 
of species in 12 genera have been observed by ourselves and 
others, but only in Robinia with any care. Psm'alea acauUs 
raises its three leaflets at night; whilst AmorpTia fruticosa^ 



* Ducharte, ' Elements de Botanique,' 1867, p. 349. 



Chap. VII. SLEEP OF LEAVES. 359 

Dalea alopecuroides, and Indigofera tinctoria depress them. 
Duchartre * states that TepJirosia caribcea is the sole example 
of "folioles couchees le long du petiole et vers la base;" but a 
similar movement occurs, as we have already seen, and shall 
again see in other cases. Wistaria sinensis, according to 
Rover, t ''abaisse les folioles qui par une disposition bizarre 
sont inclinees dans la ra^me feuille, les superieures vers le 
sommet, les inferieures vers la base du petiole commun ;" but 
the leaflets on a young plant observed by us in the green- 
house merely sank vertically downwards at night. The leaflets 
are raised in SphcBvophysa salsola, Golutea arborea, and Astra- 
galus uliginosus, but are depressed, according to Linnaeus, in 
Glycyrrhiza. The leaflets of llohinia pseudo-acacia likewise sink 
vertically down at night, but the petioles rise a little, viz., in 
one case 3°, and in another 4°. The circumnutating move- 
ments of a terminal leaflet on a rather old leaf were traced 
during two days, and were simple. The leaflet fell slowly, in a 
slightly zigzag line, from 8 a.m. to 5 p.m., and then more 
rapidly; by 7 a.m.' on the following morning it had risen to its 
diurnal position. There was only one peculiarity in the move- 
ment, namely, that on both days there was a distinct though 
small oscillation up and down between 8.30 and 10 a.m., and 
this would probably have been more strongly pronounced if 
the leaf had been younger. 

Coronilla rosea (Tribe 6). — The leaves bear 9 or 10 pairs of 
opposite leaflets, which during the day stand horizontally, with 
their midribs at right angles to the petiole. At night they rise 
up, so that the opposite leaflets come nearly into contact, and 
those on the younger leaves into close contact. At the same 
time they bend back towards the base of the petiole, until their 
midribs form with it angles of from 40° to 50° in a vertical 
plane, as here figured (Fig. 146). The leaflets, however, some- 
times bend so much back that their midribs become parallel to 
and lie on the petiole. They thus occupy a reversed position 
to what they do in several Leguminosse, for instance, in Mimosa 
pudica; but, from standing further apart, they do not overlap 



* Ducharte. ' Elements de Bo- t ' Ann. des Sciences. Nats, 
tanique,' p. 347. Bot. (5th series), ix. 1868. 

24 



360 MODIFIED CIRCUMNUTATION. Chap. VII. 

one another nearly so much as in this latter plant. The main 
petiole is curved slightly downwards during the day, but 
straightens itself at night. In three cases it rose from 3'' above 




Coronilla rosea : leaf asleep. 

the horizon at noon, to 9° at 10 p.m. ; from 11'' to 33° ; and from 
5° to 33° — the amount of angular movement in this latter case 
amounting to 28°. In several other species of Coronilla the 
leaflets showed only feeble movements of a similar kind. 

Hedysarum coronarium (Tribe 6). — The small lateral leaflets 
on plants growing out of doors rose up vertically at night, but 
the large terminal one became only moderately inclined. The 
petioles apparently did not rise at all. 

Smithia Pfundii (Tribe 6). — The leaflets rise up vertically, 
and the main petiole also rises considerably. 

Arachis hypogcea (Tribe 6). — The shape of a leaf, with its 
two pairs of leaflets, is shown at A (Fig. 147) ; and a leaf 
asleep, traced from a photograph (made by the aid of alumin- 
ium light), is given at B. The two terminal leaflets twist round 
at night until their blades stand vertically, and approach each 
other until they meet, at the same time moving a little upwards 
and backwards. The two lateral leaflets meet each other in the 
same manner, but move to a greater extent forwards, that is, in 
a contrary direction to the two terminal leaflets, which they 
partially embrace. Thus all four leaflets form together a single 
packet, with their edges directed to the zenith, and with their 
lower surfaces turned outwards. On a plant which was not 
growing vigorously the closed leaflets seemed too lieavy for the 



Chap. VII. 



SLEEP OF LEAVES. 



361 



petioles to support them in a vertical position, so that each 
night the main petiole became twisted, and all the packets were 
extended horizontally, with the lower surfaces of the leaflets on 
one side directed to the zenith in a most anomalous manner. 
This fact is mentioned solely as a caution, as it surprised as 
greatly, until we discovered that it was an anomaly. The 
petioles are inclined upwards during the day, but sink at night, 
so as to stand at about right angles with the stem. The amount 
of sinking was measured only on one occasion, and found to be 
39°. A petiole was secured to a stick at the base of the two 
terminal leaflets, and the circumnutating movement of one of 
these leaflets was traced from 6.40 a.m. to 10.40 p.m., the plant 
being illuminated from above. The temperature was 17° — 17|° 
C, and therefore rather too low. During the 16 h. the leaflet 
moved thrice up and thrice down, and as the ascending and 
descending lines did not coincide, three ellipses were formed. 

Fig. 147. 




Arachis hypogsea : A, leaf during the day, seen from vertically above ; 
B, leaf asleep, seen laterally; copied from a photograph. Figures 
much reduced. 



Desmodium gyrans (Tribe 6). — A large and full-grown leaf 
of this plant, so famous for the spontaneous movements of the 
two little lateral leaflets, is here represented (Fig, 148). The 
large terminal leaflet sleeps by sinking vertically down, whilst 
the petiole rises up. The cotyledons do not sleep, but the first- 
formed leaf sleeps equally well as the older ones. The appear- 
ance presented by a sleeping branch and one in the day-time, 
copied from two photographs, are shown at A and B (Fig. 149), 
and w^e see how at night the leaves are crowded together, as if 



362 



MODIFIED CIRCUMNUTATION. Chap. VII. 



Fig. 148. 



for mutual protection, by the rising of the petioles. The peti- 
oles of the younger leaves near the summits of the shoots rise 
up at night, so as to stand vertical and parallel to the stem ; 
whilst those on the sides were found in four 
cases to have risen respectively 46^°, 36°, 
30°, and 19-5° above the inclined positions 
which they had occupied during the day. 
For instance, in the first of these four cases 
the petiole stood in the day at 23°, and at 
night at 69^° above the horizon. In the 
evening the rising of the petioles is almost 
completed before the leaflets sink perpen- 
dicularly downwards. 

Circumnutation. — The circumnutating 
movements of four young shoots were ob- 
served during 5 h. 15 m. ; and in this time 
each completed an oval figure of small size. 
The main petiole also circumnutates rapidly, 
for in the course of 31 m. (temp,, 91° F.) it 
changed its course by as much as a rectangle 
six times, describing a figure which appar- 
ently represented two ellipses. The move- 
ment of the terminal leaflet by means of its 
sub-petiole or pulvinus is quite as rapid, or 
even more so, than that of the main petiole, 
and has much greater amplitude. Pfefler has seen * these leaf- 
lets move through an angle of 8° in the course of from 10 to 
30 seconds. 

A fine, nearly full-grown leaf on a young plant, 8 inches 
in height, with the stem secured to a stick at the base of 
the leaf, was observed from 8.30 a.m. June 22nd to 8 a.m. June 
24th. In the diagram given on page 364 (Fig. 150), the two 
curved broken lines at the base, which represent the nocturnal 
courses, ought to be prolonged far downwards. On the first 
day the leaflet moved thrice down and thrice up, and to a con- 
siderable distance laterally; the course was also remarkably 
crooked. The dots were generally made every hour; if they 




Desmodium gyrans : 
leaf seen from 
above, reduced 
to one-half natu- 
ral size. The 
minute stipules 
unusually" large. 



* 'Die Period. Beweg.,' p. 35. 



Chap. VII. 



SLEEP OF LEAVES. 



363 



had been made every few minutes all the lines would have been 
zigzag to an extraordinary degree, with here and there a loop 
formed. We may infer that this would have been the case, 
because five dots were made in the course of 31 m. (between 
12.34 and 1.5 p.m.), and we see in the upper part of the diagram 
how crooked the course here is ; if only the first and last dots 
had been joined we should have had a straight line. Exactly 
the same fact may be seen in the lines representing the course 
between 2.24 p.m. and 3 p.m., when six intermediate dots were 



Fig. 149. 




A B 

Desmodium gyrans : A, stem during the day ; B, stem with leaves 
asleep. Copied from a photograph ; figures reduced. 

made; and again at 4.46 and 4.50. But the result was widely 
different after 6 p.m., — that is, after the great nocturnal descent 
had commenced ; for though nine dots were then made in the 
course of 32 m., when these were joined (see Figure) the line 
thus formed was almost straight. The leaflets, therefore, be- 
gin to descend in the afternoon by zigzag lines, but as soon as 



364: 



HODmEB CIKCW^tlTATlOK. 



Chap. VIT. 



e»S(/ttw.2a 




6'j^'f*' 



Chap. VII. SLEEP OP LEAVES. 365 

the descent becomes rapid their whole energy is expended in 
thus moving, and their course becomes rectilinear. After the 
leaflets are completely asleep they move very little or not at all. 

Had the above plant been subjected to a higher temperature 
than 67'' — 70° F., the movements of the terminal leaflet would 
probably have been even more rapid and wider in extent than 
those shown in the diagram ; for a plant was kept for some time 
in the hot-house at from 92° — 93° F,, and in the course of 35 m. 
the apex of a leaflet twice descended and once ascended, travelling 
over a space of 1*2 inch in a vertical direction and of "82 inch 
in a horizontal direction. Whilst thus moving the leaflet also 
rotated on its own axis (and this was a point to which no atten- 
tion had been before paid), for the plane of the blade differed by 
41° after an interval of only a few minutes. Occasionally the 
leaflet stood still for a short time. There was no jerking move- 
ment, which is so characteristic of the little lateral leaflets. A 
sudden and considerable fall of temperature causes the terminal 
leaflet to sink downwards; thus a cut-off leaf was immersed in 
water at 95° F., which was slowly raised to 103° F., and after- 
wards allowed to sink to 70° F., and the sub-petiole of the ter- 
minal leaflet then curved downwards. The water was afterwards 
raised to 120° F. , and the sub-petiole straightened itself. Similar 
experiments with leaves in water were twice repeated, with 
nearly the same result. It should be added, that water raised 
to even 122° F. does not soon kill a leaf. A plant was placed 
in darkness at 8.37 a.m., and at 2 p.m. (i.e. after 5 h. 23 m), though 
the leaflets had sunk considerably, they had by no means ac- 
quired their nocturnal vertically dependent position. Pfeffer, on 
the other hand, says * that this occurred with him in from | h. 
to 2 h. ; perhaps the difference in our results may be due to 
the plant on which we experimented being a very young and 
vigorous seedling. 

The Movements of the little Lateral Leaflets.— These have been 
so often described, that we will endeavour to be as brief as pos- 
sible in giving a few new facts and conclusions. The leaflets 
sometimes quickly change their position by as much as nearly 
180°; and their sub-petioles can then be seen to become 



-^- ' Die Period. Beweg.,' p. 39. 



366 MODIFIED CIRCUMNUTATION. Chap. VII. 

greatly cnrved. They rotate on their own axes, so that their 
upper surfaces are directed to all points of the compass. The 
figure described by the apex is an irregular oval or ellipse. They 
sometimes remain stationary for a period. In these several 
respects there is uo difference, except in rapidity and extent, 
between their movements and the lesser ones performed by the 
large terminal leaflet whilst making its great oscillations. The 
movements of the little leaflets are much influenced, as is well 
known, by temperature. This was clearly shown by immersing 
leaves with motionless leaflets in cold water, which was slowly 
raised to 103° F., and the leaflets then moved quickly, describ- 
ing about a dozen little irregular circles in 40 m. By this time 
the water had become much cooler, and the movements became 
slower or almost ceased; it was then raised to 100° F., and the 
leaflets again began to move quickly. On another occasion a 
tuft of fine leaves was immersed in water at 53° F., and the 
leaflets were of course motionless. The water was raised to 99°, 
and the leaflets soon began to move ; it was raised to 105°, and 
the movements became much more rapid ; each little circle or 
oval being completed in from 1 m. 30 s. to 1 m. 45 s. There 
was, however, no jerking, and this fact may perhaps be attrib- 
uted to the resistance of the water. 

Sachs states that the leaflets do not move until the surround- 
ing air is as high as 71° — 73° F., and this agrees with our 
experience on full-grown, or nearly full-grown, plants. But the 
leaflets of young seedlings exhibit a jerking movement at much 
lower temperatures. A seedling was kept (April 16th) in a room 
for half the day when the temperature was steady at 64° F., 
and the one leaflet which it bore was continually jerking, but 
not so rapidly as in the hot-house. The pot was taken in the 
evening into a bed-room where the temperature remained at 
62° during nearly the whole night ; at 10 and 11 p.m. and at 
1 A.M. the leaflet was still jerking rapidly; at 3,30 a.m. it was 
not seen to jerk, but was observed during only a short time. It 
was, however, now inclined at a much lower angle than that 
occupied at 1 a.m. At 6.30 a.m. (temp. 61° F.) its inclination 
was still less than before, and again less at 6.45 a.m. ; by 7.40 
A.M. it had risen, and at 8.30 a.m. was again seen to jerk. This 
leaflet, therefore, was moving during the whole night, and the 



Chap. VII. SLEEP OF LEAVES. 367 

movement was by jerks up to 1 a.m. (and possibly later) and again 
at 8.30 A.M., though the- temperature was only 61° to 62° F. 
We must therefore conclude that the lateral leaflets produced 
by young plants differ somewhat in constitution from those 
on older plants. 

In the large genus Desmodium by far the greater number of 
the species are trifoliate ; but some are unifoliate, and even the 
same plant may bear uni- and trifoliate leaves. In most of the 
species the lateral leaflets are only a little smaller than the ter- 
minal one. Therefore the lateral leaflets of D. gyrans (see for- 
mer Fig. 148) must be considered as almost rudimentary. They 
are also rudimentary in function, if this expression may be used; 
for they certainly do not sleep like the full-sized terminal leaf- 
lets. It is, however, possible that the sinking down of the leaf- 
lets between 1 a.m. and 6.45 a.m., as above described, may 
represent sleep. It is well known that the leaflets go on jerk- 
ing during the early part of the night ; but my gardener observed 
(Oct. 13th) a plant in the hot-house between 5 and 5.30 a.m., the 
temperature having been kept up to 82° F., and found that all 
the leaflets were inclined, but he saw no jerking movement until 
6.55 A.M., by which time the terminal leaflet had risen and 
was awake. Two days afterwards (Oct. 15th) the same plant was 
observed by him at 4.47 a.m. (temp. 77° F.), and he found that 
the large terminal leaflets were awake, though not quite hori- 
zontal; and the only cause which we could assign for this 
anomalous wakefulness was that the plant had been kept for ex- 
perimental purposes during the previous day at an unusually high 
temperature; the little lateral leaflets were also jerking at this 
hour, but whether there was any connection between this latter 
fact and the sub-horizontal position of the terminal leaflets we 
do not know. Anyhow, it is certain that the lateral leaflets do 
not sleep like the terminal leaflets ; and in so far they may be 
said to be in a functionally rudimentary condition. They are in 
a similar condition in relation to irritability; for if a plant be 
shaken or syringed, the terminal leaflets sink down to about 45° 
beneath the horizon; but we could never detect any effect thus 
produced on the lateral leaflets ; yet we are not prepared to 
assert positively that rubbing or pricking the pulvinus produces 
no effect. 



368 MODIFIED CmCUMNUTATION. Chap. VII. 

As in the case of most rudiraentary organs, the leaflets are 
variable in size ; they often depart from their normal position 
and do not stand opposite one another; and one of the two is 
frequently absent. This absence appeared in some, but not in 
all the cases, to be due to the leaflet having become completely 
confluent with the main petiole, as might be inferred from the 
presence of a slight ridge along its upper margin, and from the 
course of the vessels. In one instance there was a vestige of 
the leaflet, in the shape of a minute point, at the further end of 
the ridge. The frequent, sudden, and complete disappearance 
of one or both of the rudimentary leaflets is a rather singular 
fact ; but it is a much more surprising one that the leaves which 
are first developed on seedling plants are not provided with 
them. Thus, on one seedling the seventh leaf above the coty- 
ledons was the first which bore any lateral leaflets, and then only 
a single one. On another seedling, the eleventh leaf first bore 
a leaflet ; of the nine succeeding leaves five bore a single lateral 
leaflet, and four bore none at all ; at last a leaf, the twenty-first 
above the cotyledons, w^as provided with two rudimentary lat- 
eral leaflets. From a widespread analogy in the animal king- 
dom, it might have been expected that these rudimentary leaflets 
w^ould have been better developed and more regularly present 
on very young than on older plants. But bearing in mind, 
firstly, that long-lost characters sometimes reappear late in life, 
and secondly, that the species of Desmodium are generally tri- 
foliate, but that some are unifoliate, the suspicion arises that 
D. gyrans is descended from a unifoliate species, and that this 
was descended from a trifoliate one ; for in this case both the 
absence of the little lateral leaflets on very young seedlings, and 
their subsequent appearance, may be attributed to reversion to 
more or less distant progenitors.* 

No one supposes that the rapid movements of the lateral 
leaflets of D. gyrans are of any use to the plant ; and why they 
should behave in this manner is quite unknown. We imagined 
that their power of movement might stand in some relation 



* Desmodium vespertilionis is rudimentary lateral leaflets. Du- 
closely allied to D. gyrans, and chartre, ' Elements dc Botanique, 
it seems only occasionally to bear 1867, p. 353. 



Chap. VII. SLEEP OF LEAVES. 369 

with their rudimentary condition, and therefore observed the 
almost rudimentary leaflets of Mimosa albida ml sensitiva (of 
which a drawing will hereafter be given, Fig. 159) ; but they 
exhibited no extraordinary movements, and at night they went 
to sleep like the full-sized leaflets. There is, however, this 
remarkable difference in the two cases : in Desmodium the pul- 
vinus of the rudimentary leaflets has not been reduced in lengtn, 
in correspondence with the reduction of the blade, to the same 
extent as has occurred in the Mimosa; and it is on the length 
and degree of curvature of the pulvinus that the amount of 
movement of the blade depends. Thus, the average length of 
the pulvinus in the large terminal leaflets of Desmodium is 
3 mm., whilst that of the rudimentary leaflets is 2*86 mm. ; so 
that they differ only a little in length. But in diameter they 
differ much, that of the pulvinus of the little leaflets being only 
0-3 mm. to 0*4 mm. ; whilst that of the terminal leaflets is 
1'33 mm. If we now turn to the Mimosa, we find that the 
average length of the pulvinus of the almost rudimentary 
leaflets is only 0*466 mm., or rather more than a quarter of the 
length of the pulvinus of the full-sized leaflets, namely, 1.66 mm. 
In this small reduction in length of the pulvinus of the rudi- 
mentary leaflets of Desmodium, we apparently have the proxi- 
mate cause of their great and rapid circumnutating movement, 
in contrast with that of the almost rudimentary leaflets of the 
Mimosa. The small size and weight of the blade, and the little 
resistance opposed by the air to its movement, no doubt also 
come into play ; for we have seen that these leaflets if immersed 
in water, when the resistance would be much greater, were pre- 
vented from jerking forwards. Why, during the reduction of 
the lateral leaflets of Desmodium, or during their reappearance 
— if they owe their origin to reversion — the pulvinus should 
have been so much less affected than the blade, whilst with the 
Mimosa the pulvinus has been greatly reduced, we do not know. 
Nevertheless, it deserves notice that the reduction of the leaflets 
in these two genera has apparently been effected by a different 
process and for a different end ; for with the Mimosa the reduc- 
tion of the inner and basal leaflets was necessary from the want 
of space ; but no such necessity exists with Desmodium, and the 
reduction of its lateral leaflets seems to have been due to the 



370 



MODIFIED CIRCUMNUTATION. Chap. VII. 



principle of compensation, in consequence of the great size of 
the terminal leaflet. 

Uraria (Tribe 6) and Centrosema (Tribe 8). — The leaflets of 
TJraria lagopus and the leaves of a Centrosema from Brazil both 
sink vertically down at night. In the latter plant the petiole at 
the same time rose 16|°. 

AmpMcarpcea monoica (Tribe 8).— The leaflets sink down 
vertically at night, and the petioles likewise fall considerably. 

Fig. 151. 




AmpMcarpsea monoica : circumnutation and nyctitropic movement of 
leaf during 48 h. ; its apex 9 inches from the vertical glass. Fig- 
ure reduced to one-third of original scale. Plant illuminated 
from above. Temp. 17i°-18r C. 



A petiole, which was carefully observed, stood during the day 
25° above the horizon and at night 32° below it; it therefore 
fell 57°. A filament was fixed transversely across the terminal 
leaflet of a fine young leaf (2J inches in length including the 
petiole), and the movement of the whole leaf was traced on a 
vertical glass. This was a bad plan in some respects, because 
the rotation of the leaflet, independently of its rising or falling, 
raised and depressed the filament; but it was the best plan for 
our special purpose of observing whether the leaf moved much 



Chap. VII. 



SLEEP OF LEAVES. 



371 



after it had gone to sleep. 
The plant had twined closely 
round a thin stick, so that the 
circumnutation of the stem 
was prevented. The move- 
ment of the leaf was traced 
during 48 h., from 9 a.m. July 
10th to 9 A.M. July 13th. In 
the figure given (Fig. 151) we 
see how complicated its course 
was on both days : during the 
second day it changed its 
course greatly 13 times. The 
leaflets began to go to sleep 
a little after 6 p.m., and by 
7,15 P.M. hung vertically down 
and were completely asleep; 
but on both nights they con- 
tinued to move from 7.15 p.m. 
to 10.40 and 10.50 p.m., quite 
as much as during the day; 
and this was the point which 
we wished to ascertain. We 
see in the figure that the great 
sinking movement late in the 
evening does not differ essen- 
tially from the circumnutation 
during the day. 

Glycine hispida (Trilje 8). 
— The three leaflets sink verti- 
cally down at night. 

Erythrina (Tribe 8). — Five 
species were observed, and 
the leaflets of all sank verti- 
cally down at night ; with E. 
"caffra and with a second un- 
named species, the petioles at 
the same time rose slightly. 
The movements of the termi- 



Fig. 152. 



6'dffaMt.8.. 



^'Jl5a.m.9^ 



2'£.Vl.\ 



Erythrina crista-galli: circumnuta- 
tion and nyctitropic movement 
of terminal leaflet, 3S inches in 
length, traced during 25 h. ; apex 
of leaf 3i inches from the verti- 
cal glass. Figure reduced to one- 
half of original scale. Plant illu- 
minated from above. Temp. 
17^-18^ C. 



372 MODIFIED CIRCUMNUTATION. Chap. VII. 

nal leaflet of E. cristagalli (with the main petiole secured to a 
gtick) were traced from 6.40 a.m., June 8th, to 8 a.m. on the 
10th. In order to observe the nyctitropic movements of this 
plant, it is necessary that it should have grown in a warm 
greenhouse, for out of doors in our climate it does not sleep. 
We see in the tracing (Fig. 152) that the leaflet oscillated twice 
up and down between early morning and noon; it then fell 
greatly, afterwards rising till 3 p.m. At this latter hour the 
great nocturnal fall commenced. On the second day (of which 
the tracing is not given) there was exactly the same double 
oscillation before noon, but only a very small one in the after- 
noon. On the third morning the leaflet moved laterally, which 
was due to its beginning to assume an oblique position, as 
seems invariably to occur with the leaflets of this species as 
they grow old. On both nights after the leaflets were asleep 
and hung vertically down, they continued to move a little both 
up and down, and from side to side. 

Erythrina caffra. — A filament was fixed transversely across 
a terminal leaflet, as we wished to observe its movements when 
asleep. The plant was placed in the morning of June 10th 
under a skylight, where the light was not bright; and we do 
not know whether it was owing to this cause or to the plant 
having been disturbed, but the leaflet hung vertically down all 
day ; nevertheless it circumnutated in this position, describing 
a figure which represented two irregular ellipses. On the next 
day it circumnutated in a greater degree, describing four irregu- 
lar ellipses, and by 3 p.m. had risen into a horizontal position. 
By 7.15 P.M. it was asleep and vertically dependent, but con- 
tinued to circumnutate as long as observed, until 11 p.m. 

Erythrina corallodendron. — The movements of a terminal 
leaflet were traced. During the second day it oscillated four 
times up and four times down between 8 a.m. and 4 p.m., after 
which hour the great nocturnal fall commenced. On the third 
day the movement was equally great in amplitude, but was 
remarkably simple, for the leaflet rose in an almost perfectly 
straight line from 6.50 a.m. to 3 p.m., and then sank down in an 
equally straight line until vertically dependent and asleep. 

Apios tuberosa (Tribe 8). — The leaflets sink vertically down 
at nio-ht. 



Chap. VII. SLEEP OF LEAVES. 3Y3 

Phaseolus vulgaris (Tribe 8). — The leaflets likewise sink ver- 
tically down at night. In the greenhouse the petiole of a young 
leaf rose 16°, and that of an older leaf 10" at night. With 
plants growing out of doors the leaflets apparently do not sleep 
until somewhat late in the season, for on the nights of July llth 
and 12th none of them were asleep ; whereas on the night of 
August 15th the same plants had most of their leaflets verti- 
cally dependent and asleep. With PJi. caracalla and Hernan- 
desii, the primary unifoliate leaves and the leaflets of the sec- 
ondary trifoliate leaves sink vertically down at night. This 
holds good with the secondary trifoliate leaves of Ph. Pox- 
hurghii, but it is remarkable that the primary unifoliate leaves, 
which are much elongated, rise at night from about 20° to 
about 60° above the horizon. With older seedlings, however, 
having the secondary leaves just developed, the primary leaves 
stand in the middle of the day horizontally, or are deflected 
a little beneath the horizon. In one such case the primary 
leaves rose from 26° beneath the horizon at noon, to 20° above 
it at 10 P.M. ; whilst at this same hour the leaflets of the 
secondary leaves were vertically dependent. Here, then, we 
have the extraordinary case of the primary and se'condary 
leaves on the same plant moving at the same time in opposite 
directions. 

We have now seen that the leaflets in the six genera of 
Phaseoleae observed by us (with the exception of the primary 
leaves of Phaseolus Boxburghii) all sleep in the same manner, 
namely, by sinking vertically down. The movements of the 
petioles were observed in only three of these genera. They rose 
in Centrosema and Phaseolus, and sunk in Amphicarpgea. 

Sophora chrijsophylla (Tribe 10). — The leaflets rise at night, 
and are at the same time directed towards the apex of the leaf, 
as in Mimosa pudica. 

Gansalpinia^ Hcematoxylon^ Oleditschia, Poinciana. — The leaf- 
lets of two species of Caesalpinia (Tribe 13) rose at night. 
With Hmmntoxylon Gampechianum (Tribe 13) the leaflets move 
forwards at night, so that their midribs stand parallel to the 
petiole, and their now vertical lower surfaces are turned out- 
wards (Fig. 153). Tlie petiole sinks a little. In Gleditschia, if 
we understand correctly Duchartre's description, and in Poin- 



374 



MODIFIED CmCUMNUTATIOX. Chap. VII 



nana Gilliesii (both belonging to Tribe 13), the leaves behave 
in the same manner. 

Fig. 153. 




A B 

Hxmatoxylon Campechianum : A, branch during daytime ; B, branch 
with leaves asleep, reduced to two-thirds of natural scale. 

Cassia (Tribe 14). — The nyctitropic movements of the leaves 
in many species in this genus are closely alike, and are highly 
complex-. They were first briefly described by Linnaeus, and since 
by Duchartre. Our observations were made chiefly on C. flari- 
tunda * and coryrribosa, but several other species were casually 
observed. The horizontally extended leaflets sink down verti- 
cally at night ; but not simply, as in so many other genera, for 
each leaflet rotates on its own axis, so that its lower surface 
faces outwards. The upper surfaces of the opposite leaflets are 
thus brought into contact with one another beneath the petiole, 
and are well protected (Fig. 154). The rotation and other move- 
ments are effected by means of a well-developed puhdnus at the 
base of each leaflet, as could be plainly seen when a straight 
narrow black line had been painted along it during the day. 
The two terminal leaflets in the daytime include rather less than 
a right angle ; but their divergence increases greatly whilst they 
sink downwards and rotate, so that they stand laterally at night, 



*I am informed by ^Mr. Dyer 
that Mr. Bentham believes that 
C. floribunda (a common green- 
house bush) is a hybrid raised in 
France, and that it comes very 



near to C. Irevigaia. It is no doubt 
the same as the form described by 
Lindley CBot. Eeg.,' Tab. 1422) 
as C. Herhertiana. 



Chap. VII. SLEEP OP LEAVES. 375 

as may be seen in the figure. Moreover, they move somewhat 
backwards, so as to point towards the base of the petiole. In 
one instance we found that the midrib of a terminal leaflet 




A B 

Cassia corymbosa : A, plant during day ; B, same plant at night. 
Both figures copied from photographs. 

formed at night an angle of 36°, with a line dropped perpen- 
dicularly from the end of the petiole. The second pair of leaflets 
25 



3Y6 



MODIFIED CIRCUMNUTATION. Chap. VII. 



likewise moves a little backwards, but less than the terminal 
pair ; and the third pair moves vertically downwards, or even a 
little forwards. Thus all the leaflets, in those species which bear 
only 3 or 4 pairs, tend to form a single packet, with their upper 
surfaces in contact, and their lower surfaces turned outwards. 
Lastly, the main petiole rises at night, but with leaves of differ- 
ent ages to very different degrees, namely, some rose through an 
angle of only 13°, and others as much as 41°. 

Cassia calUantha — The leaves bear a large number of leaflets, 
which move at night in nearly the same manner as just described : 
but the petioles apparently do not rise, and one which was care- 
fully observed certainly fell 3°. 

Fig. 155. 




Cassia pubescens : A, upper part of plant during the day ; B, same plant 
at niglit. Figures reduced from photographs. 

Cassia pubescens. — The chief difference in the nyctitropic 
movements of this species, compared with those of the former 
species, consists in the leaflets not rotating nearly so much; 



Chap. VII. SLEEP OF LEAVES. 377 

therefore their lower surfaces face but little outwards at night. 
The petioles, which during the day are inclined only a little 
above the horizon, rise at night in a remarkable manner, and 
stand nearly or quite vertically. This, together with the de- 
pendent position of the leaflets, makes the whole plant wonder- 
fully compact at night. In the two foregoing figures, copied 
from photographs, the same plant is represented awake and 
asleep (Fig. 155), 'and we see how different is its appearance. 

Cassia mimosoides. — At night the numerous leaflets on each 
leaf rotate on their axes, and their tips move towards the apex 
of the leaf; they thus become imbricated with their lower sur- 
faces directed upwards, and with their midribs almost parallel 
to the petiole. Consequently, this species differs from all the 
others seen by us, with the exception of the following one, in 
the leaflets not sinking down at night. A petiole, the movement 
of which was measured, rose 8° at night. 

Cassia Barclay ana. — The leaflets of this Australian species are 
numerous, very narrow, and almost linear. At night they rise up 
a little, and also move towards the apex of the leaf. For instance, 
two opposite leaflets which diverged from one another during 
the day at an angle of 104°, diverged at night only 72°; so that 
each had risen 16° above its diurnal position. The petiole of a 
young leaf rose at night 34°, and that of an older leaf 19°. 
Owing to the slight movement of the leaflets and the consider- 
able movement of the petiole, the bush presents a different 
appearance at night to what it does by day ; yet the leaves can 
hardly be said to sleep. 

The circumnutating movements of the leaves of C. Jioridunda, 
calliantha, and pubescens were observed, each during three or four 
days ; they were essentially alike, those of the last-named species 
being the simplest. The petiole of C. floribunda was secured to 
a stick at the base of the two terminal leaflets, and a filament 
was fixed along the midrib of one of them. Its movements were 
traced from 1 p.m. on August 13th to 8.30 a.m. 17th; but those 
during the last 2 h. are alone given in Fig. 156. From 8 a.m. on 
each day (by which hour the leaf had assumed its diurnal posi- 
tion) to 2 or 3 P.M., it either zigzagged or circumnutated over 
nearly the same small space; at between 2 and 3 p.m. the great 
evening fall commenced. The lines representing this fall and 



378 



MODIFIED CIRCUMNUTATION. Chap. VII. 



Fig. 156. 




Cassia florihunda : circumnutation and 
nyctitropic movement of a terminal 
leaflet iU inch in length) traced 
from 8.30 a.m. to samehour on fol- 
lowing morning. Apex of leaflet 
5i inches from the vertical glass. 
Main petiole 3| inches long. Temp. 
16°-17^° C. Figure reduced to one- 
half of the original scale. 



the early morning rise are 
oblique, owing to the pecul- 
iar manner in which the 
leaflets sleep, as already 
described. After the leaflet 
was asleep at 6 p.m., and 
whilst the glass filament 
hung perpendicularly down, 
the movement of its apex 
was traced until 10.30 p.m. ; 
and during this whole time 
it swayed from side to 
side, completing more than 
one ellipse. 

Bauhinia (Tribe 15). — 
The nyctitropic movements 
of four species were alike, 
and were highly peculiar. 
A plant raised from seed 
sent us from South Brazil 
by Fritz Mtiller, was more 
especially observed. The 
leaves are large and deeply 
notched at their ends. At 
night the two halves rise 
up and close completely to- 
gether, like the opposite 
leaflets of many Legumi- 
nosse. With very young 
plants the petioles rise con- 
siderably at the same time ; 
one, which was inclined at 
noon 45° above the horizon, 
at night stood at 75°; it 
thus rose 30° ; another rose 
34°. Whilst the two halves 
of the leaf are closing, the 
midrib at first sinks verti- 
cally downwards and after- 



Chap. VII. SLEEP OP LEAVES. 379 

wards bends backwards, so as to pass close along one side of 
its own upwardly inclined petiole; the midrib being thus 
directed towards the stem or axis of the plant. The angle 
which the midrib formed with the horizon was measured in 
one case at different hours: at noon it stood horizontally; late 
in the evening it depended vertically; then rose to the opposite 
side, and at 10.15 p.m. stood at only 27° beneath the horizon, 
being directed towards the stem. It had thus travelled througli 
153°. Owing to this movement — to the leaves being folded — 
and to the petioles rising, the whole plant is as much more 
compact at night than during the day, as a fastigiate Lombardy 
poplar is compared with any other species of poplar. It is 
remarkable that when our plants had grown a little older, viz., 
to a height of 2 or 3 feet, the petioles did not rise at night, 
and the midribs of the folded leaves were no longer bent back 
along one side of the petiole. We have noticed in some other 
genera that the petioles of very young plants rise much more 
at night than do those of older plants. 

Tamarindus Indica (Tribe 16). — The leaflets approach or 
meet each other at night, and are all directed towards the apex 
of the leaf. They thus become imbricated with their midribs 
parallel to the petiole. The movement is closely similar to 
that of Haematoxylon (see former Fig. 153), but more striking 
from the greater number of the leaflets. 

Adenanthera, Prosopis, and Heptunia (Tribe 20) . — With Ade- 
nanthera pavonia the leaflets turn edgeways and sink at night. 
In Prosopis they turn upwards. With Neptunia oleracea the 
leaflets on the opposite sides of the same pinna come into 
contact at night and are directed forwards. The pinnae them- 
selves move downwards, and at the same time backwards or 
towards the stem of the plant. The main petiole rises. 

Miw,osa pudica (Tribe 20). — This plant has been the subject 
of innumerable observations; but there are some points in rela- 
tion to our subject which have not been sufl[iciently attended 
to. At night, as is well known, the opposite leaflets come into 
contact and point towards the apex of the leaf; they thus be- 
come neatly imbricated with their upper surfaces protected. The 
four pinnsQ also approach each otlier closely, and the whole leaf 
is thus rendered very compact. The main petiole sinks down- 



380 



MODIFIED CIRCUMNUTATION. Chap. VII. 



8^3ffa.m. 



wards during the day till late in the evening, and rises until 
very early in the morning. The stem is continually circumnu- 
tating at a rapid rate, though not to a wide extent. Some very 
young plants, kept in darkness, were observed during two days, 

and although subjected to a 
rather low temperature of 
57°_59° F., the stem of one 
described four small ellipses 
in the course of 12 h. We 
shall immediately see that the 
main petiole is likewise con- 
tinually circum nutating, as is 
each separate pinna and each 
separate leaflet. Therefore, 
if the movement of the apex 
of any one leaflet were to be 
traced, the course described 
would be compounded of the 
movements of four separate 
parts. 

A filament had been fixed 
on the previous evening, lon- 
gitudinally to the main peti- 
ole of a nearly full - grown, 
highly sensitive leaf (four 
inches in length), the stem 
having been secured to a stick 
at its base ; and a tracing was 
made on a vertical glass in 
the hot-house under a high 
temperature. In the figure 
given (Fig. 157), the first dot 
was made at 8.30 a.m. Auo:ust 



TSS'tLm^ 




ey.m.2* 



jr^nu3^ 



Mimosa pudica : circumnutation and 

nyctitropic movement of main 

petiole, traced during 34 h. 30 m. 

the 3rd. During 13 h. on 
the first day the petiole moved thrice downwards and twice 
upwards. Within the same length of time on the second day, 
it moved five times downwards and four times upwards. As 
the ascending and descending lines do not coincide, the petiole 



Chap. VII. SLEEP OF LEAVES. 381 

manifestly circumnutates ; the great evening fall and nocturnal 
rise being an exaggeration of one of the circumuutations. It 
should, however, be observed that the petiole fell much lower 
down in the evenings than could be seen on the vertical glass 
or is represented in the diagram. After 7 p.m. on the 3rd 
(when the last dot in Fig. 157 was made) the pot was carried 
into a bed-room, and the petiole was found at 12.50 a.m. (i.e. 
after midnight) standing almost upright, and much more highly- 
inclined than it was at 10.40 p.m. When observed again at 4 
A.M. it had begun to fall, and continued falling till 6.15 a.m., 
after which hour it zigzagged and again circumnutated. Similar 
observations were made on another petiole, with nearly the 
same result. 

On two other occasions the movement of the main petiole 
was observed every two or three minutes, the plants being kept 
at a rather high temperature, viz., on the first occasion at 
77°— 81° F., and the filament then described 2i ellipses in 69 m. 
On the second occasion, when the temperature was 81° — 86° F., 
it made rather more than 3 ellipses in 67 m. Therefore, 
Fig. 157, though now sufficiently complex, would have been 
incomparably more so, if dots had been made on the glass every 
2 or 3 minutes, instead of every hour or half-hour. Although 
the main petiole is continually and rapidly describing small 
ellipses during the day, yet after the great nocturnal rising 
movement has commenced, if dots are made every 2 or 3 
minutes, as was done for an hour between 9.30 and 10.30 p.m. 
(temp. 84** F.), and the dots are then joined, an almost abso- 
lutely straight line is the result. 

To show that the movement of the petiole is in all proba- 
bility due to the varying turgescence of the pulvinus, and not 
to growth (in accordance with the conclusions of Pfejffer), a very 
old leaf, with some of its leaflets yellowish and hardly at all 
sensitive, was selected for observation, and the plant was kept 
at the highly favourable temp, of 80° F. The petiole fell from 
8 A.M. till 10.15 a.m. ; it then rose a little in a somewhat zigzag 
line, often remaining stationary, till 5 p.m., when the great 
evening fall commenced, which was continued till at least 
10 P.M. By 7 a.m. on the following morning it had risen to the 
same level as on the previous morning, and then descended in 



382 MODIFIED CIRCUMNUTATION. Chap. VII. 

a zigzag line. But from 10.30 a.m. till 4.15 p.m. it remained 
almost motionless, all power of movement being now lost. The 
petiole, therefore, of this very old leaf, which must have long 
ceased growing, moved periodically; but instead of circum- 
nutating several times during the day, it moved only twice 
down and twice up in the course of 24 h., with the ascending 
and descending lines not coincident. 

It has already been stated that the pinnae move independently 
of the main petiole. The petiole of a leaf was fixed to a cork 
support, close to the point whence the four pinnae diverge, with 
a short fine filament cemented longitudinally to one of the two 
terminal pinnae, and a graduated semicircle was placed close 
beneath it. By looking vertically down, its angular or lateral 
movements could be measured with accuracy. Between noon 
and 4.15 p.m. the pinna changed its position to one side by only 
7"; but not continuously in the same direction, as it moved 
four times to one side, and three times to the opposite side, 
in one instance to the extent of 16^. This pinna, therefore, 
circumnutated. Later in the evening the four pinnae approach 
each other, and the one which was observed moved inwards 
59° between noon and 6.45 p.m. Ten observations were made 
in the course of 2 h. 30 m. (at average intervals of 14 m.), 
between 4.35 and 6.45 p.m. ; and there was now, when the leaf 
was going to sleep, no swaying from side to side, but a steady 
inward movement. Here therefore there is in the evening the 
same conversion of a circumnutating into a steady movement 
in one direction, as in the case of the main petiole. 

It has also been stated that each separate leaflet circumnu- 
tates, A pinna was cemented with shellac on the summit of a 
little stick driven firmly into the ground, immediately beneath 
a pair of leaflets, to the midribs of both of which excessively 
fine glass filaments were attached. This treatment did not 
injure the leaflets, for they went to sleep in the usual manner, 
and long retained their sensitiveness. The movements of one 
of them were traced during 49 h., as shown in Fig. 158. On 
the first day the leaflet sank down till 11.30 a.m., and then rose 
till late in the evening in a zigzag line, indicating circumnuta- 
tion. On the second day, when more accustomed to its new 
state, it oscillated twice up and twice down during the 34 h. 



Chap. VII. 



SLEEP OF LEAVES. 



383 



This plant was subjected to a rather low temperature, viz., 
62° — 64° F. ; had it been kept warmer, no doubt the movements 
of the leaflet would have been much more rapid and compli- 

Fig. 158. 




/©•4fOVtr«y^«* 



ical 



Mimosa piuKca : circumnutation and nyctitropic movement of a leaflet 
(with pinna secured), traced on a vertical glass, from 8 a.m. Sept. 
14th to 9 A.M. 16th. 



Gated. It may be seen in the diagram that the ascending and 
descending lines do not coincide; but the large amount of lat- 
eral movement in the evening is the result of the leaflets bending 



384 MODIFIED CIRCUMNUTATION. Chap. YII. 

towards the apex of the leaf when going to sleep. Another 
leaflet was casually observed, and found to be continually cir- 
cumnutating during the same length of time. 

The circumnutation of the leaves is not destroyed by their 
being subjected to moderately long continued darkness; but 
the proper periodicity of their movements is lost. Some very 
young seedlings were kept during two days in the dark (temp. 
57° — 59° Y.), except when the circumnutation of their stems 
was occasionally observed ; and on the evening of the second 
day the leaflets did not fully and properly go to sleep. The pot 
was then placed for three days in a dark cupboard, under nearly 
the same temperature, and at the close of this period the leaf- 
lets showed no signs of sleeping, and were only slightly sensi- 
tive to a touch. On the following day the stem was cemented 
to a stick, and the movements of two leaves were traced on a 
vertical glass during 73 h. The plants w^ere still kept in the 
dark, excepting that at each observation, which lasted 3 or 4 
minutes, they were illuminated by two candles. On the third 
day the leaflets still exhibited a vestige of sensitiveness when 
forcibly pressed, but in the evening they showed no signs of 
sleep. Nevertheless, their petioles continued to circumnutate 
distinctly, although the proper order of their movements in 
relation to the day and night was wholly lost. Thus, one leaf 
descended during the first two nights (i.e. between 10 p.m. and 
7 A.M. next morning) instead of ascending, and on the third 
night it moved chiefly in a lateral direction. The second leaf 
behaved in an equally abnormal manner, moving laterally dur- 
ing the first night, descending greatly during the second, and 
ascending to an unusual height during the third night. 

With plants kept at a high temperature and exposed to 
the light, the most rapid circumnutating movement of the 
apex of a leaf which was observed, amounted to ^i^ of an 
inch in one second; and this would have equalled | of an 
inch in a minute, had not the leaf occasionally stood still. 
The actual distance travelled by the apex (as ascertained by 
a measure placed close to the leaf) was on one occasion nearly 
f of an inch in a vertical direction in 15 m. ; and on another 
occasion | of an inch in 60 m. ; but there was also some lateral 
movement. 



Chap. VII. 



SLEEP OF LEAVES. 



385 



Mimosa albida.* — The leaves of this plant, one of which is here 
figured (Fig. 159) reduced, to f of the natural size, present some 
interesting peculiarities. It consists of a long petiole bearing 
only two pinnae (here represented as rather more divergent 
than is usual), each with two pairs of leaflets. But the inner 
basal leaflets are greatly reduced in size, owing probably to the 
want of space for their full development, so that they may be 
considered as almost rudimentary. They vary somewhat in 
size, and both occasionally disapjjear, or only one. Neverthe- 
less, they are not in the least rudimentary in function, for they 
are sensitive, extremely heliotropic, circumnutate at nearly the 
same rate as the fully developed leaflets, and assume when 

Fig. 159. 




Mimosa albida : leaf seen from vertically above. 



asleep exactly the same position. With M. pudica the inner 
leaflets at the base and between the pinnae are likewise much 
shortened and obliquely truncated ; this fact was well seen in 
some seedlings of M. pudica, in which the third leaf above the 
cotyledons bore only two pinnae, each with only 3 or 4 pairs of 
leaflets, of which the inner basal one was less than half as long 
as its fellow; so that the whole leaf resembled pretty closely 
that of M. albida. In this latter species the main petiole termi- 



* Mr. Thistleton Dyer informs 
us that this Peruvian plant (which 
was sent to us from Kew) is con- 
sidered by Mr. Bentham ( ' Trans. 



Linn. Soc.,' vol. xxx. p. 390) to 
be "the species or variety which 
most commonly represents the M. 
sensitiva of our gardens. 



386 MODIFIED CIRCUMNUTATION. Cuap. VII. 

nates in a little point, and on each side of this there is a pair 
of minute, flattened, lancet-shaped projections, hairy on their 
margins, which drop off and disappear soon after the leaf is 
fully developed. There can be hardly a doubt that these little 
projections are the last and fugacious representatives of an 
additional pair of leaflets to each pinna ; for the outer one is 
twice as broad as the inner one, and a little longer, viz., y^^ of 
an inch, whilst the inner one is only |^ long. Now if the basal 
pair of leaflets of the existing leaves were to become rudimen- 
tary, we should expect that the rudiments would still exhibit 
some trace of their present great inequality of size. The con- 
clusion that the pinnae of the parent- form of M. alhida possessed 
at least three pairs of leaflets, instead of, as at present, only two, 
is supported by the structure of the flrst true leaf; for this 
consists of a simple petiole, often bearing three pairs of leaflets. 
This latter fact, as well as the presence of the rudiments, both 
lead to the conclusion that M. alhida is descended from a form 
the leaves of which bore more than two pairs of leaflets. The 
second leaf above the cotyledons resembles in all respects the 
leaves on fully developed plants. 

When the leaves go to sleep, each leaflet twists half round, 
so as to present its edge to the zenith, and comes into close 
contact with its fellow. The pinnae also approach each other 
closely, so that the four terminal leaflets come together. The 
large basal leaflets (with the little rudimentary ones in contact 
with them) move inwards and forwards, so as to embrace the 
outside of the united terminal leaflets, and thus all eight leaflets 
(the rudimentary ones included) form together a single vertical 
packet. The two pinnae at the same time that they approach 
each other sink downwards, and thus instead of extending hori- 
zontally in the same line with the main petiole, as during the 
day, they depend at night at about 45°, or even at a greater 
angle, beneath the horizon. The movement of the main petiole 
seems to be variable ; we have seen it in the evening 27° lower 
than during the day; but sometimes in nearly the same posi- 
tion. Nevertheless, a sinking movement in the evening and 
a rising one during the night is probably the normal course, 
for this was well-marked in the petiole of the first-formed true 
leaf. 



Chap. VII. SLEEP OF LEAVES. 387 

The circumnutation of the main petiole of a young leaf was 
traced during 2f days, and was considerable in extent, but less 
complex than that of M. pudica. The movement was much 
more lateral than is usual with circumnutating leaves, and this 
was the sole peculiarity which it presented. The apex of one of 
the terminal leaflets was seen under the microscope to travel ^^ 
of an inch in 3 minutes. 

Mimosa marginata. — The opposite leaflets rise up and ap- 
proach each other at night, but do not come into close contact, 
except in the case of very young leaflets on vigorous shoots. 
Full-grown leaflets circumnutate during the day slowly and 
on a small scale. 

SchranJcia uncinata (Tribe 20). — A leaf consists of two or three 
pairs of pinnae, each bearing many small leaflets. These, when 
the plant is asleep, are directed 'forwards and become imbricated. 
The angle between the two terminal pinnae was diminished at 
night, in one case by 15° ; and they sank almost vertically down- 
wards. The hinder pairs of pinna likewise sink downwards, 
but do not converge, that is, move towards the apex of the leaf. 
The main petiole does not become depressed, at least during the 
evening. In this latter respect, as well as in the sinking of the 
pinnae, there is a marked difference between the nyctitropic 
movements of the present plant and of Mimosa pudica. It 
should, however, be added that our specimen was not in a very 
vigorous condition. The pinnae of Schrankia aculeata also sink 
at night. 

Acacia Famesiana (Tribe 22). — The different appearance pre- 
sented by a bush of this plant when asleep and awake is won- 
derful. The same leaf in the two states is shown in the following 
figure (Fig. 160). The leaflets move towards the apex of the 
pinna and become imbricated, and the pinnae then look like bits 
of dangling string. The following remarks and measurements 
do not fully apply to the small leaf here figured. The pinnae 
move forwards and at the same time sink downwards, whilst 
the main petiole rises considerably. With respect to the degree 
of movement : the two terminal pinnae of one specimen formed 
together an angle of 100° during the day, and at night of only 
38°, 80 each had moved 31° forwards. The penultimate pinnae 
during the day formed together an angle of 180°, that is, they 



388 



MODIFIED CIRCUMNUTATION. Chap. VII. 



stood in a straight line opposite one another, and at night each 
had moved 65° forwards. The basal pair of pinnas were directed 
during the day, each about 31° backwards, and at night 38° 
forwards, so each had moved 59° forwards. But the pinnas at 
the same time sink greatly, and sometimes hang almost perpen- 



Fig. 160. 




Acacia Farnesiana , 



B 

A. leaf durinj 



the day ; B, the same leaf at night. 



dicularly downwards. The main petiole, on the other hand, 
rises much: by 8.30 p.m. one stood 34° higher than at noon, 
and by 6.40 a.m. on the following morning it was still higher 
by 10° ; shortly after this hour the diurnal sinking move- 
ment commenced. The course of a nearly full-grown leaf was 
traced during 14 h. ; it was strongly zigzag, and apparently 
represented five ellipses, with their longer axes differently 
directed. 

AJhizzia lopJiantha (Tribe 23). — The leaflets at night come 
into contact with one another, and are directed towards the 
apex of the pinna. The pinnae approach one another, but re- 



Chap. VIT. SLEEP OF LEAVES. 389 

main in the same plane as during the day; and in this respect 
they differ much from those of the above Schrankia and Acacia. 
The main petiole rises but little. The first-formed leaf above 
the cotyledons bore 11 leaflets on each side, and these slept like 
those on the subsequently formed leaves ; but the petiole of this 
first leaf was curved downwards during the day and at night 
straightened itself, so that the chord of its arc then stood 16" 
higher than in the day-time. 

Melaleuca ericcefolia (Myrtaceae). — According to Bouch6 
('Bot. Zeit.,' 1874, p. 359) the leaves sleep at night, in nearly 
the same manner as those of certain species of Pimelia. 

(Enothera mollissima (Onagrariese). — According to Linnaeus 
(' Somnus Plantarum '), the leaves rise up vertically at night. 

Passiflora gracilis (Passifloracse). — The young leaves sleep 
by their blades hanging vertically downwards, and the whole 
length of the petiole then becomes somewhat curved down- 
wards. Externally no trace of a pulvinus can be seen. The 
petiole of the uppermost leaf on a young shoot stood at 10.45 
A.M. at 33'' above the horizon ; and at 10.30 p.m., when the blade 
was vertically dependent, at only 15°, so the petiole had fallen 
18°. That of the next older leaf fell only 7°. From some 
unknown cause the leaves do not always sleep properly. The 
stem of a plant, which had stood for some time before a north- 
east window, was secured to a stick at the base of a young leaf, 
the blade of which was inclined at 40° below the horizon. 
From its position the leaf had to be viewed obliquely, conse- 
quently the vertically ascending and descending movements 
appeared when traced oblique. On the first day (Oct. 13th) 
the leaf descended in a zigzag line until late in the evening; 
and by 8.15 a.m. on the 13th had risen to nearly the same level 
as on the previous morning. A new tracing was now begun 
(Fig. 161). The leaf continued to rise until 8.50 a.m., then 
moved a little to the right, and afterwards descended. Be- 
tween 11 a.m. and 5 p.m. it circumnutated, and after the latter 
hour the great nocturnal fall commenced. At 7.15 p.m. it 
depended vertically. The dotted line ought to have been pro- 
longed much lower down in the figure. By 6.50 a.m. on the 
following morning (14th) the leaf had risen greatly, and con- 
tinued to rise till 7.50 a.m., after which hour it redescended. 



390 MODIFIED CIRCUMNUTATION. Chap. VII. 

It should be observed that the lines traced on this second morn- 
ing would have coincided with and confused those previously 
traced, had not the pot been slided a very little to the left. In 
the evening (14th) a mark was placed behind the filament 




Passiflora gracilis : circumnutation and nyctitropic movement of leaf, 
traced on vertical glass, from 8.20 a.m. Oct. 13th to 10 A.M. 14th. 
Figure reduced to two-thirds of original scale. 

attached to the apex of the leaf, and its movement was care- 
fully traced from 5 p.m. to 10.15 p.m. Between 5 and 7.15 p.m. 
the leaf descended in a straight line, and at the latter hour it 
appeared vertically dependent. But between 7.15 and 10.15 
P.M. the line consisted of a succession of steps, the cause of 
which we could not understand ; it was, however, manifest that 
the movement was no longer a simple descending one. 

Siegesbechia orientalis (Compositae). — Some seedlings were 
raised in the middle of winter and kept in the hot-house ; they 
flowered, but did not grow well, and their leaves never showed 
any signs of sleep. The leaves on other seedlings raised in May 
were horizontal at noon (June 22nd), and depended at a con- 
siderable angle beneath the horizon at 10 p.m. In the case of 



Chap. VII. 



SLEEP OF LEAVES. 



391 



four youngish leaves, which were from 2 to 2^ inches in length, 
these angles were found to be 50°, 56°, 60°, and 65°. At the 
end of August, when the plants had grown to a height of 10 to 
11 inches, the younger leaves were so much curved downw^ards 
at night that they might truly be said to be asleep. This is one 
of the species which must be well illuminated during the day 
in order to sleep, for on two occasions when plants were kept 
all day in a room with north-east windows, the leaves did not 
sleep at night. The same cause probably accounts for the 

Fig. 162. 




Nicotiana glauca: shoots witli leaves expanded during the day, and 
asleep at night. Figures copied from photographs, and reduced. 



leaves on our seedlings raised in the dead of the winter not 
sleeping. Professor Pfeffer informs us that the leaves of an- 
other species (S. JoruUensis ?) hang vertically down at night. 

20 



392 



MODIFIED CIRCUMNUTATION. Chap. VII. 



Ipomcea ccerulea and purpurea (Convolvulacese). — The leaves 
on very young plants, a foot or two in height, are depressed at 
night to between 68° and 80° beneath the horizon ; and some 
hang quite vertically downwards. On the following morning 

they again rise into 



Fig. 163. 



lW.?5>«iJSf 



wrd^p.m.m*. 



^iS^ 



8^*a.m. 



J3^ 



itk 



Nicotiana tabacum : circumnutation and nyc- 
ti tropic movement of a leaf (5i inches in 
length), traced on a vertical glass, from 
3 P.M. July 10th to 8.10 a.m. 13th. Apex 
of leaf 4 inches from glass. Temp. 17i°- 
18i° C. Figure reduced to one-half origi- 
nal scale. 



a horizontal position. 
The petioles become 
at night downwardly 
curved, either through 
their entire length or 
in the upper part 
alono ; and this appar- 
ently causes the de- 
pression of the blade. 
It seems necessary that 
the leaves should be 
well illuminated dur- 
ing the day in order to 
sleep, for those M^hich 
stood on the back of 
a plant before a north- 
east window did not 
sleep. 

Nicotiana tabacum 
(var. Virginian) and 
glauca (Solanese). — 
The young leaves of 
both these species 
sleep by bending verti- 
cally upwards. Fig- 
ures of two shoots of 
N. glauca, awake and 
asleep (Fig. 162), are 
given on p. 391 : one 
of the shoots, from 
which the photographs 
were taken, was acci- 
dentally bent to one 
side. 



Chap. VII. SLEEP OF LEAVES. 393 

At the base of the petiole of JV. tdbacum on the outside, 
there is a mass of cells, which are rather smaller than elsewhere, 
and have their longer axes differently directed from the cells of 
the parenchyma, and may therefore be considered as forming a 
sort of pulvinus. A young plant of N. tabacum was selected, 
and the circumnutation of the fifth leaf above the cotyledons 
was observed during three days. On the first morning (July 
10th) the leaf fell from 9 to 10 a.m., which is its normal course, 
but rose during the remainder of the day ; and this no doubt 
was due to its being illuminated exclusively from above; for 
properly the evening rise does- not commence until 3 or 4 p.m. 
In the figure as given on p. 386 (Fig. 163) the first dot was 
made at 3 p.m. ; and the tracing was continued for the follow- 
ing 65 h. When the leaf pointed to the dot next above that 
marked 3 p.m., it stood horizontally. The tracing is remarkable 
only from its simplicity and the straightness of the lines. The 
leaf each day described a single great ellipse ; for it should be 
observed that the ascending and descending lines do not coin- 
cide. On the evening of the 11th the leaf did not descend 
quite so low as usual, and it now zigzagged a little. The diur- 
nal sinking movement had already commenced each morning 
by 7 A.M. The broken lines at the top of the figure, represent- 
ing the nocturnal vertical position of the leaf, ought to be pro- 
longed much higher up. 

Mirahilis longijior^ and jalapa (Nyctagineae). — The first pair 
of leaves above the cotyledons, produced by seedlings of both 
these species, were considerably divergent during the day, and 
at night stood up vertically in close contact with one another. 
The two upper leaves on an older seedling were almost horizontal 
by day, and at night stood up vertically, but were not in close 
contact, owing to the resistance offered by the central bud. 

Polygonum avieulare (Polygoneae).— Professor Batalin informs 
us that the young leaves rise up vertically at night. This is 
likewise the case, according to Linngeus, with several species 
of Amaranthus (Amaranthaceae) ; and we observed a sleep move- 
ment of this kind in one member of the genus. Again, with 
Chenopodium album (Chenopodieae), the upper young leaves of 
some seedlings, about 4 inches in height, were horizontal or 
sub-horizontal during the day, and at 10 p.m. on March 7th 



394 MODIFIED CIRCUMNUTATION. Chap. VII. 

were quite, or almost quite, vertical. Other seedlings raised in 
the greenhouse during the winter (Jan. 28th) were observed day 
and night, and no difference could be perceived in the position 
of their leaves. According to Bouche ('Bot. Zeitung,' 1874, 
p. 359) the leaves of Pimelia linoidts and spectaMUs (Thymeleae) 
sleep at night. 

Euphorbia jacquinioejlora (Euphorbiaceae). — Mr. Lynch called 
our attention to the fact that the young leaves of this plant 
sleep by depending vertically. The third leaf from the sum- 
mit (March 11th) was inclined during the day 30° beneath the 
horizon, and at night hung vertically down, as did some of 
the still younger leaves. It rose up to its former level on the 
following morning. The fourth and fifth leaves from the summit 
stood horizontally during the day, and sank down at night only 
38°. The sixth leaf did not sensibly alter its position. The 
sinking movement is due to the downward curvature of the 
petiole, no part of which exhibits any structure like that of 
a pulvinus. Early on the morning of June 7th a filament was 
fixed longitudinally to a young leaf (the third from the summit, 
and 2|- inches in length), and its movements were traced on 
a vertical glass during 73 h., the plant being illuminated from 
above through a skylight. Each day the leaf fell in a nearly 
straight line from 7 a.m. to 5 p.m., after which hour it was so 
much inclined downwards that the movement could no longer 
be traced ; and during the latter part of each night, or early in 
the morning, the leaf rose. It therefore circumnutated in a 
very simple manner, making a single large ellipse every 34 h., 
for the ascending and descending lines did not coincide. On 
each successive morning it stood at a less height than on the 
previous one, and this was probably due, partly to the increasing 
age of the leaf, and partly to the illumination being insuflScient ; 
for although the leaves are very slightly heliotropic, yet, accord- 
ing to Mr. Lynch's and our own observations, their inclination 
during the day is determined by the intensity of the light. On 
the third day, by which time the extent of the descending 
movement had much decreased, the line traced was plainly 
much more zigzag than on any previous day, and it appeared 
as if some of its powers of movement were thus expended. At 
10 P.M. on June 7th, when the leaf depended vertically, its 



Chap. VII. SLEEP OF LEAVES. 395 

movements were observed by a mark being placed behind it, 
and the end of the attached filament was seen to oscillate slowly 
and slightly from side to side, as well as upwards and down- 
wards. 

PTiyllanthns Niruri (Euphorbiacese). — The leaflets of this 
plant sleep, as described by Pfeffer,* in a remarkable manner, 
apparently like those of Cassia, for they sink downwards at 
night and twist round, so that their lower surfaces are turned 
outwards. They are furnished, as might have been expected 
from this complex kind of movement, with a pulvinus. 

Gymn^ospekms. 

Finns Nordmanniana (Coniferse). — M. Chatin states t that the 
leaves which are horizontal during the day, rise up at night, so 
as to assume a position almost perpendicular to the branch from 
which they arise ; we presume that he here refers to a horizontal 
branch. He adds: " En meme temps, ce mouvement d'erection 
est accompagne d'un mouvement de torsion imprime a la partie 
basilaire de la feuille, et pouvant souvent parcourir un arc de 
90 degres." As the lower surfaces of the leaves are white, 
whilst the upper are dark green, the tree presents a widely 
different appearance by day and night. The leaves on a small 
tree in a pot did not exhibit with us any nyctitropic move- 
ments. We have seen in a former chapter that the leaves of 
Pinus 'pinaster and Austriaca are continually circumnutating. 

MOKOCOTYLEDOIfS. 

Thalia dealbata (Cannaceae). — The leaves of this plant sleep 
by turning vertically upwards; they are furnished with a well- 
developed pulvinus. It is the only instance known to us of 
a very large leaf sleeping. The blade of a young leaf, which 
was as yet only 13^ inches in length and 6^^ in breadth, formed 
at noon an angle with its tall petiole of 121°, and at night stood 
vertically in a line with it, and so had risen 59°. The actual 
distance travelled by the apex (as measured by an orthogonic 
tracing) of another large leaf, between 7.30 a.m. and 10 p.m., 



*• 'Die Period, Beweg.,' p. 159. 

t 'Comptes Rendus,' Jan. 1876, p. 171. 



396 MODIFIED CmCUMNUTATION. Chap. VII. 

was 10^ inches. The circumnutation of two young and dwarfed 
leaves, arising amongst the taller leaves at the base of the plant, 
was traced on a vertical glass during two days. On the first day 
the apex of one, and on the second day the apex of the other 
leaf, described between 6.40 a.m. and 4 p.m. two ellipses, the 
longer axes of which were extended in very different directions 
from the lines representing the great diurnal sinking and noc- 
turnal rising movement. 

Maranta arundinacea (Cannacese). — The blades of the leaves, 
which are furnished with a pulvinus, stand horizontally during 
the day or between 10° and 20° above the horizon, and at night 
vertically upwards. They therefore rise between 70° and 90° at 
night. The plant was placed at noon in the dark in the hot- 
house, and on the following day the movements of the leaves 
were traced. Between 8.40 and 10.30 a.m. they rose, and then 
fell greatly till 1.37 p.m. But by 3 p.m. they had again risen a 
little, and continued to rise during the rest of the afternoon and 
night ; on the following morning they stood at the same level as 
on the previous day. Darkness, therefore, during a day and a 
half does not interfere with the periodicity of their movements. 
On a warm but stormy evening, the plant whilst being brought 
into the house, had its leaves violently shaken, and at night not 
one went to sleep. On the next morning the plant was taken 
back to the hot-house, and again at night the leaves did not 
sleep ; but on the ensuing night they rose in the usual manner 
between 70° and 80°. This fact is analogous with what we 
have observed with climbing plants, namely, that much agita- 
tion checks for a time their power of circumnutation; but the 
effect in this instance was much more strongly marked and 
prolonged. 

Colocasi antiquoTuma {Caladium esculentum^ Hort.) (Aroidese). 
— The leaves of this plant sleep by their blades sinking in the 
evening, so as to stand highly inclined, or even quite vertically 
with their tips pointing to the ground. They are not provided 
with a pulvinus. The blade of one stood at noon 1° beneath the 
horizon; at 4.20 p.m., 20°; at 6 p.m., 43°; at 7.20 p.m., 69°; and 
at 8.30 P.M., 68°; so it had now begun to rise; at 10.15 p.m. it 
stood at 65°, and on the following early morning at 11° beneath 
the horizon. The circumnutation of another young leaf (with 



Chap. VII. 



SLEEP OF LEAVES. 



397 



its petiole only 3J inches, and the blade 4 inches in length), was 
traced on a vertical glass during 48 h. ; it was dimly illuminated 
through a skylight, and this seemed to disturb the proper perio- 
dicity of its movements. Nevertheless, the leaf fell greatly 
during both afternoons, till either 7.10 p.m. or 9 p.m., when it 
rose a little and moved laterally. By an early hour on both 
mornings, it had assumed its diurnal position. The well-marked 
lateral movement for a short time in the early part of the night, 
was the only interesting fact which it presented, as this caused 
the ascending and descending lines not to coincide, in accord- 
ance with the general rule with circumnutating organs. The 
movements of the leaves of this plant are thus of the most 
simple kind; and the tracing is not worth giving. We have 
seen that in another genus of the Aroideae, namely, Pistia, the 
leaves rise so much at 
night that they may almost ^^' 

be said to sleep. 

Strephium Jloribundwn * 
(Graminese) . — The oval 
leaves are provided with 
a pulvinus, and are ex- 
tended horizontally or 
declined a little beneath 
the horizon during the 
day. Those on the up- 
right culms simply rise up 
vertically at night, so that 
their tips are directed to- 
wards the zenith (Fig. 164) . 
Horizontally extended 
leaves arising from much 
inclined or almost hori- 
zontal culms, move at night so that their tips point towards 
the apex of the culm, with one lateral margin directed towards 
the zenith ; and in order to assume this position the leaves have 
to twist on their own axes through an angle of nearly 90°. 





Strephium floribundum : culms with 
leaves during the day, and when 
asleep at night. Figures reduced. 



* A. Brongniart first observed 
that the leaves of this plant and 
of Marsilea sleep : see ' Bull, de 



la Soc. Bot. de France,' torn. 
1860, p. 470. 



398 



MODIFIED CIECUMNUTATION. Chap. VII. 



165. 



Strephium floribundum : circnmnu- 
tation and nyctitropic movement 
of a leaf, traced from 9 a.m. June 
26th to 8.45 A.M. 27th ; filament 
fixed along the midrib. Apex of 
leaf 8i inches from the vertical 
glass ; plant illuminated from 
above. Temp. 23^°-24i° C. 



Thus the surface of the blade 
always stands vertically, what- 
ever may be the position of 
the midrib or of the leaf as a 
whole. 

The circumnutation of a 
young leaf (2*3 inches in length 
was traced during 48 h. (Fig. 
165). The movement was re- 
markably simple ; the leaf de- 
scended from before 6.40 a.m. 
until 3 or 2.50 p.m., and then 
rose so as to stand vertically at 
about 6 P.M., descending again 
late in the night or in the very 
early morning. On the second 
day the descending line zig- 
zagged slightly. As usual, the 
ascending and descending lines 
did not coincide. On another 
occasion, when the tempera- 
ture was a little higher, viz., 
34°-26|'' C, a leaf was ob- 
served 17 times between 8.50 
A.M. and 13.16 p.m. ; it 
changed its course by as much 
as a rectangle six times in this 
interval of 3 h. 26 m., and 
described two irregular tri- 
angles and a half. The leaf, 
therefore, on this occasion cir- 
cumnutated rapidly and in a 
complex manner. 

ACOTYLEDONS. 

Marsilea quadrifoliata (Mar- 

sileacese). — The shape of a leaf, 

expanded horizontally during 

the day, is shown at A (Fig. 



Chap. VII. 



SLEEP OF LEAVES. 



399 



166). Each leaflet is provided with a well-developed pulvinus. 
When the leaves sleep, the two terminal leaflets rise up, twist 
half round and come into contact with one another (B), and are 

Fig. 166. 




ABC 

Marsilea quadrifoliata : A, leaf during the day, seen from vertically 
above ; B, leaf beginning to go to sleep, seen laterally ; C, the 
same asleep. Figures reduced to one-half of natural scale. 

afterwards embraced by the two lower leaflets (C) ; so that the 
four leaflets with their lower surfaces turned outwards form a 
vertical packet. The curvature of the summit of the petiole of 
the leaf figured asleep, is merely accidental. The plant was 



G'p.m, 




JD'Ji'o,^, 



Marsilea quadrifoliata: circumnutation and nyctitropic movement of 
leaflet traced on vertical glass, during nearly 24 h. Figure re- 
duced to two-thirds of original scale. Plant kept at rather too 
low a temperature. 

brought into a room, where the temperature was only a little 
above 60° F., and the movement of one of the leaflets (the 
petiole having been secured) was traced during 24 h. (Fig. 
167). The leaf fell from the early morning till 1.50 p.m., and 



400 MODIFIED CIRCUMNUTATION. Chap. VII. 

then rose till 6 p.m., wlien it was asleep. A vertically depend- 
ent glass filament was now fixed to one of the terminal and 
inner leaflets; and part of the tracing in Fig. 167, after 6 p.m., 
shows that- it continued to sink, making one zigzag, until 
10.40 P.M. At 6.45 A.M. on the following morning, the leaf 
was awaking, and the filament pointed above the vertical glass, 
but by 8.35 a.m. it occupied the position shown in the figure. 
The diagram differs greatly in appearance from most of those 
previously given; and this is due to the leaflet twisting and 
moving laterally as it approaches and comes into contact with 
its fellow. The movement of another leaflet, when asleep, 
was traced between 6 p.m. and 10.35 p.m., and it clearly cir- 
cumnutated, for it continued for two hours to sink, then rose, 
and then sank still lower than it was at 6 p.m. It may be 
seen in the preceding figure (167) that the leaflet, when the 
plant was subjected to a rather low temperature in the house, 
descended and ascended during the middle of the day in a 
somewhat zigzag line; but when kept in the hot-house from 
9 A.M. to 3 P.M. at a high but varying temperature (viz., between 
72° and 83° F.) a leaflet (with the petiole secured) circumnutated 
rapidly, for it made three large vertical ellipses in the course of 
the six hours. According to Brongniart, Marsilea pubescens sleeps 
like the present species. These plants are the sole cryptogamic 
ones known to sleep. 

Siimmary and Co7ichiding Bemarhs on the Nycti- 
tropic or Sleep-movements of Leaves. — That these move- 
ments are in some manner of high importance to the 
plants which exhibit them, few will dispute who have 
observed how complex they sometimes are. Thus with 
Cassia, the leaflets which are horizontal during the day 
not only bend at night vertically downwards with the 
terminal pair directed considerably backwards, but they 
also rotate on their own axes, so that their lower sur- 
faces are turned outwards. The terminal leaflet of Meli- 
lotns likewise rotates, by which movement one of its 
lateral edges is directed upwards, and at the same time 



Chap. VII. SUMMARY ON SLEEP OF LEAVES. 401 

it moves either to the left or to the right, until its upper 
surface comes into contact with that of the lateral leaflet 
on the same side, which has likewise rotated on its own 
axis. With Arachis, all four leaflets form together dur- 
ing the night a single vertical packet; and to effect 
this the two anterior leaflets have to move upwards and 
the two posterior ones forwards, besides all twisting 
on their own axes. In the genus Sida the leaves of 
some species move at night through an angle of 90° 
upwards, and of others through the same angle down- 
wards. We have seen a similar difference in the nycti- 
tropic movements of the cotyledons in the genus Oxalis. 
In Lupinus, again, the leaflets move either upwards or 
downwards; and in some species, for instance, L. hitens, 
those on one side of the star-shaped leaf move up, and 
those on the opposite side move down; the interme- 
diate ones rotating on their axes ; and by these varied 
movements, the wliole leaf forms at night a vertical 
star instead of a horizontal one, as during the day. 
Some leaves and leaflets, besides moving either upwards 
or downwards, become more or less folded at night, 
as in Bauhinia and in some species of Oxalis. The 
positions, indeed, which leaves occupy when asleep are 
almost infinitely diversified ; they may point either ver- 
tically upwards or downwards, or, in the case of. leaflets, 
towards the apex or towards the base of the leaf, or in 
any intermediate position. They often rotate at least as 
much as 90° on their own axes. The leaves which arise 
from upright and from horizontal or much inclined 
branches on the same plant, move in some few cases 
in a different manner, as with Porlieria and Strephium. 
The whole appearance of many plants is wonderfully 
changed at night, as may be seen with Oxalis, and still 
more plainly with Mimosa. A bush of Acacia Farnesi- 
ana appears at night as if covered with little dangling 



402 MODIFIED CIRCUMNUTATION. Chap. YII. 

bits of string instead of leaves. Excluding a few genera 
not seen by ourselves, about which we are in doubt, 
and excluding a few others the leaflets of which rotate 
at night, and do not rise or sink much, there are 37 
genera in which the leaves or leaflets rise, often mov- 
ing at the same time towards the apex or towards the 
base of the leaf, and 32 genera in which they sink at 
night. 

The nycti tropic movements of leaves, leaflets, and 
petioles are effected in two different ways ; firstly, by 
alternately increased growth on their opposite sides, pre- 
ceded by increased turgescence of the cells ; and sec- 
ondly by means of a pulvinus or aggregate of small 
cells, generally destitute of chlorophyll, which become 
alternately more turgescent on nearly opposite sides ; 
and this turgescence is not followed by growth except 
during the early age of the plant. A pulvinus seems 
to be formed (as formerly shown) by a group of cells 
ceasing to grow at a very early age, and therefore does 
not differ essentially from the surrounding tissues. The 
cotyledons of some species of Trifolium are provided 
with a pulvinus, and others are destitute of one, and so 
it is with the leaves in the genus Sida. We see also 
in the same genus gradations in the state of the devel- 
opment of the pulvinus ; and in Xicotiana we have 
what may probably be considered as the commencing 
development of one. The nature of the movement is 
closely similar, whether a pulvinus is absent or present, 
as is evident from many of the diagrams given in this 
chapter. It deserves notice that when a pulvinus is 
present, the ascending and descending lines hardly ever 
coincide, so that ellipses are habitually described by the 
leaves thus provided, whether they are young or so old 
as to have quite ceased growing. This fact of ellipses 
being described, shows that the alternately increased 



Chap. VII. SUMMARY ON SLEEP OF LEAVES. 403 

turgescence of the cells does not occur on exactly opposite 
sides of the pulvinus, any more than the increased growth 
which causes the movements of leaves not furnished 
with pulvini. When a pulvinus is present, the nycti- 
tropic movements are continued for a very much longer 
period than when such do not exist. This has been 
amply proved in the case of cotyledons, and Pfeiler has 
given observations to the same effect with respect to 
leaves. We have seen that a leaf of Mimosa pudica con- 
tinued to move in the ordinary manner, though some- 
what more simply, until it withered and died. It may 
be added that some leaflets of TrifoUum pi'atense were 
pinned open during 10 days, and on the first evening 
after being released they rose up and slept in the usual 
manner. Besides the long continuance of the move- 
ments when effected by the aid of a pulvinus (and this 
appears to be the final cause of its development), a twist- 
ing movement at night, as Pfeffer has remarked, is 
almost confined to leaves thus provided. 

It is a very general rule that the first true leaf, 
though it may differ somewhat in shape from the leaves 
on the mature plant, yet sleeps like them ; and this oc- 
curs quite independently of the fact whether or not the 
cotyledons themselves sleep, or whether they sleep in the 
same manner. But with Phaseolus Roxhurghii the first 
unifoliate leaves rise at night almost sufficiently to be 
said to sleep, whilst the leaflets of the secondary trifoliate 
leaves sink vertically at night. On young plants of Sida 
rlioinbifolia^ only a few inches in height, the leaves did 
not sleep, though on rather older plants they rose up 
vertically at night. On the other hand, the leaves on 
very young plants of Cytisus fragrans slept in a con- 
spicuous manner, whilst on old and vigorous bushes kept 
in the greenhouse, the leaves did not exhibit any plain 
nyctitropic movement. In the genus Lotus the basal 



404 MODIFIED CIRCUMNUTATION. Chap. VII. 

stipule-like leaflets rise up vertically at night, and are 
provided with pulvini. 

As already remarked, when leaves or leaflets change 
their position greatly at night and by complicated move- 
ments, it can hardly be doubted that these must be in 
some manner beneficial to the plant. If so, we must ex- 
tend the same conclusion to a large number of sleeping 
plants ; for the most complicated and the simplest nycti- 
tropic movements are connected together by the finest 
gradations. But owing to the causes specified in the be- 
ginning of this chapter, it is impossible in some few cases 
to determine whether or not certain movements should 
be called nyctitropic. Generally, the position wliich the 
leaves occupy at night indicates with sufficient clearness, 
that the benefit thus derived, is the protection of their 
upper surfaces from radiation into the open sky, and in 
many cases the mutual protection of all the parts from 
cold by their being brought into close approximation. 
It should be remembered thfit it was proved in the last 
chapter, that leaves compelled to remain extended hori- 
zontally at night, suffered much more from radiation 
than those which were allowed to assume their normal 
vertical position. 

The fact of the leaves of several plants not sleeping 
unless they have been well illuminated during the day, 
made us for a time doubt whether the protection of their 
upper surfaces from radiation was in all cases the final 
cause of their well-pronounced nyctitropic movements. 
But we have no reason to suppose that the illumination 
from the open sky, during even the most clouded day, 
is insufficient for this purpose ; and we should bear in 
mind that leaves which are shaded from being seated low 
down on the plant, and which sometimes do not sleep, 
are likewise protected at night from full radiation. 
Nevertheless, we do not wish to deny that there may 



Chap. VII. SUMMARY ON SLEEP OF LEAVES. 405 

exist cases in which leaves change their position con- 
siderably at night, without their deriving any benefit 
from such movements. 

Although with sleeping plants the blades almost 
always assume at night a vertical, or nearly vertical 
position, it is a point of complete indifference whether 
the apex, or the base, or one of the lateral edges, is 
directed to the zenith. It is a rule of wide generality, 
that whenever there is any difference in the degree of 
exposure to radiation between the upper and the lower 
surfaces of leaves and leaflets, it is the upper which is 
the least exposed, as may be seen in Lotus, Cytisus, 
Trifolium, and other genera. In several species of 
Lupinus the leaflets do not, and apparently from their 
structure cannot, place themselves vertically at night, 
and consequently their upper surfaces, though highly 
inclined, are more exposed than the lower ; and here we 
have an exception to our rule. But in other species of 
this genus the leaflets succeed in placing themselves 
vertically ; this, however, is effected by a very unusual 
movement, namely, by the leaflets on the opposite sides 
of the same leaf moving in opposite directions. 

It is again a very common rule that when leaflets 
come into dose contact with one another, they do so by 
their upper surfaces, which are thus best protected. In 
some cases this may be the direct result of their rising 
vertically ; but it is obviously for the protection of the 
upper surfaces that the leaflets of Cassia rotate in so 
wonderful a manner whilst sinking downwards ; and that 
the terminal leaflet of Melilotus rotates and moves to one 
side until it meets the lateral leaflet on the same side. 
When opposite leaves or leaflets sink vertically down 
without any twisting, their lower surfaces approach each 
other and sometimes come into contact ; but this is the 
direct and inevitable result of their position. With 



406 MODIFIED CIRCUMNUTATION. Chap. VII. 

many species of Oxalis the lower surfaces of the adjoin- 
ing leaflets are pressed together, and are thus better pro- 
tected than the upper surfaces ; but this depends merely 
on each leaflet becoming folded at night so as to be able 
to sink vertically downwards. The torsion or rotation 
of leaves and leaflets, which occurs in so many cases, 
apparently always serves to bring their upper surfaces 
into close approximation with one another, or with other 
parts of the plant, for their mutual protection. We see 
this best in such cases as those of Arachis Mimosa alhida 
and Marsilea, in which all the leaflets form together at 
night a single vertical packet. If with Mimosa ]^udica 
the opposite leaflets had merely moved upwards, their 
upper surfaces would have come into contact and been 
well protected ; but as it is, they all successively move 
towards the apex of the leaf ; and thus not only their 
upper surfaces are protected, but the successive pairs 
become imbricated and mutually protect one another as 
well as the petioles. This imbrication of the leaflets of 
sleeping plants is a common phenomenon. 

The nyctitropic movement of the blade is generally 
effected by the curvature of the uppermost part of the 
petiole, which has often been modified into a pulvinus ; 
or the whole petiole, when short, may be thus modified. 
But the blade itself sometimes curves or moves, of which 
fact Bauhinia offers a striking instance, as the two halves 
rise up and come into close contact at night. Or the 
blade and the upper part of the petiole may both move. 
Moreover, the petiole as a whole commonly either rises 
or sinks at night. This movement is sometimes large : 
thus the petioles of Cassia pulescens stand only a little 
above the horizon during the day, and at nigKt rise up 
almost, or quite, perpendicularly. The petioles of the 
younger leaves of Desmodium gyrans also rise up ver- 
tically at night. On the other hand, with Amphicar- 



Chap. VII. SUMMARY ON SLEEP OF LEAVES. 407 

paea, the petioles of some leaves sank down as much as 
57° at night; with Arachis they sank 39°, and then stood 
at right angles to the stem. Generally, when the rising 
or sinking of several petioles on the same plant was 
measured, the amount differed greatly. This is largely 
determined by the age of the leaf : for instance, the peti- 
ole of a moderately old leaf of Desmodium gyrans rose 
only 46°, whilst the young ones rose up vertically ; that 
of a young leaf of Cassia floribunda rose 41°, whilst that 
of an older leaf rose only 12°. It is a more singular fact 
that the age of the plant sometimes influences greatly 
the amount of movement ; thus with some young seed- 
lings of a Bauhinia the petioles rose at night 30° and 34°, 
whereas those on these same plants, when grown to a 
height of 2 or 3 feet, hardly moved at all. The position 
of the leaves on the plant as determined by the light, 
seems also to influence the amount of movement of the 
petiole ; for no other cause was apparent why the peti- 
oles of some leaves of Melilotus officinalis rose as much 
as 59°, and others only 7° and 9° at night. 

In the case of many plants, the petioles move at 
night in one direction and the leaflets in a directly oppo- 
site one. Thus, in three genera of Phaseolese the leaflets 
moved vertically downwards at night, and the petioles 
rose in two of them, whilst in the third they sank. Spe- 
cies in the same genus often differ widely in the move- 
ments of their petioles. Even on the same plant of 
Lupinus puhescens some of the petioles rose 30°, others 
only 6°, and others sank 4° at night. The leaflets of 
Cassia Bardayana moved so little at night that they 
could not be said to sleep, yet the petioles of some young 
leaves rose as much as 34°. These several facts appar- 
ently indicate that the movements of the petioles are not 
performed for any special purpose ; though a conclusion 
of this kind is generally rash. When the leaflets sink 
27 



408 MODIFIED CIRCUMNUTATION. Chap. VII. 

vertically down at night and the petioles rise, as often 
occurs, it is certain that the upward movement of the 
latter does not aid the leaflets in placing themselves in 
their proper position at night, for they have to move 
through a greater angular space than would otherwise 
have been necessary. 

Notwithstanding what has just been said, it may be 
strongly suspected that in some cases the rising of the 
petioles, when considerable, does beneficially serve the 
plant by greatly reducing the surface exposed to radiation 
at night. If the reader will compare the two drawings 
(Fig. 155, p. 376) of Cassia piihescens^ "copied from pho- 
tographs, he will see that the diameter of the plant at 
night is about one-third of ^what it is by day, and there- 
fore the surface exposed to radiation is nearly nine times 
less. A similar conclusion may be deduced from the 
drawings (Fig. 149, p. 363) of a branch awake and asleep 
of Desmodium gyrans. So it was in a very striking man- 
ner with young plants of Bauhinia, and with Oxalis 
Ortegesii. 

We are led to an analogous conclusion with respect 
to the movements of the secondary petioles of certain 
pinnate leaves. The pinnae of Mimosa pudica converge 
at night ; and thus the imbricated and closed leaflets on 
each separate pinnae are all brought close together into a 
single bundle, and mutually protect one another, with 
a somewhat smaller surface exposed to radiation. With 
Alhizzia lophantha the pinnae close together in the same 
manner. Although the pinnae of Acacia Farnesiana do 
not converge much, they sink downwards. Those of 
Neptunia oleracea likewise move downwards, as well as 
backwards, towards the base of the leaf, whilst the main 
petiole rises. With Schrankia, again, the pinnae are 
depressed at night. Now in these three latter cases, 
though the pinnge do not mutually protect one another 



Chap. VI I. SUMMARY ON SLEEP OF LEAVES. 409 

at night, yet after having sunk down they expose, as 
does a dependent sleeping leaf, much less surface to the 
zenith and to radiation than if they had remained 
horizontal. 

Any one who had never observed continuously a 
sleeping plant, would naturally suppose that the leaves 
moved only in the evening when going to sleep, and in 
the morning when awaking ; but he would be quite mis- 
taken, for we have found no exception to the rule that 
leaves which sleep continue to move during the whole 
twenty-four hours; they move, however, more quickly 
when going to sleep and when awaking than at other 
times. That they are not stationary during the day is 
shown by all the diagrams given, and by the many more 
which were traced. It is troublesome to observe the 
movements of leaves in the middle of the night, but this 
was done in a few cases ; and tracings were made during 
the early part of the night of the movements, in the case 
of Oxalis, Amphicarpsea, two species of Erythrina, a 
Cassia, Passiflora, Euphorbia and Marsilea; and the 
leaves after they had gone to sleep, were found to be in 
constant movement. When, however, opposite leaflets 
come into close contact with one another or with the 
stem at night, they are, as we believe, mechanically pre- 
vented from moving, but this point "was not sufficiently 
investigated. 

When the movements of sleeping leaves are traced 
during twenty-four hours, the ascending and descending 
lines do not coincide, except occasionally and by accident 
for a short space ; so that with many plants a single 
large ellipse is described during each twenty-four hours. 
Such ellipses are generally narrow and vertically directed, 
for the amount of lateral movement is small. That there 
is some lateral movement is shown by the ascending and 
descending lines not coinciding, and occasionally, as with 



410 MODIFIED CIRCUMNUTATION. Chap. VII. 

Desmodium gyrans and Tlialia dealhata\ it was strongly 
marked. In the case of Melilotus the ellipses described 
by the terminal leaflet during the day are laterally ex- 
tended, instead of vertically, as is nsiial ; and this fact 
evidently stands in relation with the terminal leaflet 
moving laterally when it goes to sleep. With the ma- 
jority of sleeping plants the leaves oscillate more than 
once up and down in the twenty-four hours; so that 
frequently two ellipses, one of moderate size, and one of 
very large size which includes the nocturnal movement, 
are described within the twenty-four hours. For in- 
stance, a leaf which stands vertically up during the 
night will sink in the morning, then rise considerably, 
again sink in the afternoon, and in the evening reascend 
and assume its vertical nocturnal position. It will thus 
describe, in the course of the twenty-four hours, two 
ellipses of unequal sizes. Other plants describe within 
the same time, three, four, or five ellipses. Occasionally 
the longer axes of the several ellipses extend in different 
directions, of which Acacia Farnesiana offered a good 
instance. The following cases will give an idea of the 
rate of movement : Oxalis acetosella completed two 
ellipses at the rate of 1 h. 25 m. for each ; Marsilea 
quadrifoliata^ at the rate of 2h.; Trifolium suMerraneum, 
one in 3 h. 30 m. ; and Arachis hypogcea, in 4 h. 50 m. 
But the number of ellipses described within a given time 
depends largely on the state of the plant and on the con- 
ditions to which it is exposed. It often happens that a 
single ellipse may be described during one day, and two 
on the next. Erytlirina corallodendron made four 
ellipses on the first day of observation and only a single 
one on the third, apparently owing to having been kept 
not sufficiently illuminated and perhaps not warm 
enough. But there seems likewise to be an innate tend- 
ency in different species of the same genus to make a 



CiiAP. VII. SUMMARY ON SLEEP OF LEAVES. 411 

different number of ellipses in the twenty-four hours : 
the leaflets of Trifoliuvi repens made only one ; those 
of T. reswpinatum two, and those of T. subterraneum 
three in this time. Again, the leaflets of Oxalis Plu- 
mierii made a single ellipse ; those of 0. hujjleuri- 
folia^ two ; those of 0. Valdiviana^ two or three ; and 
those of 0. acetosella^ at least five in the twenty-four 
hours. 

The line followed by the apex of a leaf or leaflet, 
whilst describing one or more ellipses during the day, 
is often zigzag, either throughout its whole course or 
only during the morning or evening: Kobinia offered 
an instance of zigzagging confined to the morning, and 
a similar movement in the evening is shown in the 
diagram (Fig. 126) given under Sida. The amount of 
the zigzag movement depends largely on the plant being 
placed under highly favourable conditions. But even 
under such favourable conditions, if the dots which 
mark the position of the apex are made at considerable 
intervals of time, and the dots are then joined, the course 
pursued will still appear comparatively simple, although 
the number of the ellipses will be increased ; but if dots 
are made every two or three minutes and these are 
joined, the result often is that all the lines are strongly 
zigzag, many small loops, triangles, and other figures 
being also formed. This fact is shown in two parts of 
the diagram (Fig. 150) of the movements of Desinodium. 
gyraiis. StrepMumjloribundmyi^ observed under a high 
temperature, made several little triangles at the rate 
of 43 m. for each. Mimosa pudica^ similarly observed, 
described three little ellipses in 67 m. ; and the apex 
of a leaflet crossed -g-^-g- of an inch in a second, or 0-12 
inch in a minute. The leaflets of Averrhoa made a 
countless number of little oscillations when the temper- 
ature was high and the sun shining. The zigzag move- 



412 MODIFIED CIRCUMNUTATION. Chap. VII. 

inent may in all cases be considered as an attempt to 
form small loops, which are drawn out by a prevailing 
movement in some one direction. The rapid gyrations 
of the little lateral leaflets of Desmodium belong to the 
same class of movements, somewhat exaggerated in ra- 
pidity and amplitude. The jerking movements, with a 
small advance and still smaller retreat, apparently not 
exactly in the same line, of the hypocotyl of the cabbage 
and of the leaves of Dionaea, as seen under the micro- 
scope, all probably come under this same head. We may 
suspect that we here see the energy which is freed during 
the incessant chemical changes in progress in the tissues, 
converted into motion. Finally, it should be noted that 
leaflets and probably some leaves, whilst describing their 
ellipses, often rotate slightly on their axes ; so that the 
plane of the leaf is directed first to one and then to another 
side. This was plainly seen to be the case with the large 
terminal leaflets of Desmodium, Erythrina and Amphi- 
carpaea, and is probably common to all leaflets provided 
with apulvinus. 

With respect to the periodicity of the movements of 
sleeping leaves, Pfeffer * has so clearly shown that this 
depends on the daily alternations of light and darkness, 
that nothing farther need be said on this head. But we 
may recall the behaviour of Mimosa in the North, where 
the sun does not set, and the complete inversion of the 
daily movements by artificial light and darkness. It has 
also been shown by us, that although leaves subjected to 
darkness for a moderately long time continue to cir- 
cumnutate, yet the periodicity of their movements is soon 
greatly disturbed, or quite annulled. The presence of 
light or its absence cannot be supposed to be the direct 
cause of the movements, for these are wonderfully diver- 



"••■" 'Die Periodisclien Bewegungen der Blattorgane,' 1875, p. 30, 
et passim. 



Chap. VII. SUMMARY ON SLEEP OF LEAVES. 413 

sified even with the leaflets of the same leaf, although all 
have of course been similarly exposed. The movements 
depend on innate causes, and are of an adaptive nature. 
The alternations of light and darkness merely give notice 
to the leaves that the period has arrived for them to 
move in a certain manner. We may infer from the fact 
of several plants (Tropaeolum, Lupinus, &c.) not sleep- 
ing unless they have been well illuminated during the 
day, that it is not the actual decrease of light in the 
evening, but the contrast between the amount at this 
hour and during the early part of the day, which ex- 
cites the leaves to modify their ordinary mode of cir- 
cumnutation. 

As the leaves of most plants assume their proper 
diurnal position in the morning, although light be 
excluded, and as the leaves of some plants continue to 
move in the normal manner in darkness during at least 
a whole day, we may conclude that the periodicity of 
their movements is to a certain extent inherited.* The 
strength of such inheritance differs much in different 
species, and seems never to be rigid; for plants have 
been introduced from all parts of the world into our 
gardens and greenhouses ; and if their movements had 
been at all strictly fixed in. relation to the alternations of 
day and night, they would have slept in this country at 
very different hours, which is not the case. Moreover, it 



*PfefFer denies such inherit- habit of winter and summer 
ance ; he attributes ( ' Die Period. wheat to grow best at different 
Bewegungen,' pp. 30-56) the pe- seasons; for this habit is h)st 
riodicity when prolonged for a after a few years, like the move- 
day or two in darkness, to " Nach- ments of leaves in darkness after 
wirkung," or the after-effects of a few days. No doubt some effect 
light and darkness. But we are un- must be produced on the seeds by 
able to follow his train of i-eason- the long-continued cultivation of 
ing. There does not seem to be the parent-plants under different 
any more reason for attributing climates, but no one probably 
such movements to this cause would call this the "'Nachwirk- 
than, for instance, the inherited ung " of the climates. 



414 MODIFIED CmCUMNUTATION. Chap. VII. 

has been observed that sleeping plants in their native 
homes change their times of sleep with the changing 



We may now turn to the systematic list (p. 323). 
This contains the names of all the sleeping plants 
known to us, though the list undoubtedly is very imper- 
fect. It may be premised that, as a general rule, all the 
species in the same genus sleep in nearly the same 
manner. But there are some exceptions ; in several 
large genera including many sleeping species (for in- 
stance, Oxalis), some do not sleep. One species of 
Melilotus sleeps like a Trifolium, and therefore very 
differently from its congeners ; so does one species of 
Cassia. In the genus Sida, the leaves either rise or fall 
at night ; and with Lupinus they sleep in three different 
methods. Eeturning to the list, the first point which 
strikes us, is that there are many more genera amongst 
the Leguminosae (and in almost every one of the Legu- 
minous tribes) than in all the other families put together ; 
and we are tempted to connect this fact with the great mo- 
bility of the stems and leaves in this family, as shown by 
the large number of climbing species which it contains. 
Next to the Leguminosse come the Malvacea3, together 
with some closely allied families. But by far the most 
important point in the list, is that we meet with sleeping 
plants in 28 families, in all the great divisions of the 
Phanerogamic series, and in one Cryptogam. ISTow, al- 
though it is probable that with the Legnminosae the 
tendency to sleep may have been inherited from one 
or a few progenitors and possibly so in the cohorts of 
the Malvales and Chenopodiales, yet it is manifest that 
the tendency must have been acquired by the several 
genera in the other families, quite independently of one 



Pfeffer, ' Die Period. Beweguugen,' p. 46. 



Chap. VII. SUMMARY ON SLEEP OF LEAVES. 415 

another. Hence the question naturally arises, how has 
this been possible? and the answer, we cannot doubt, 
is that leaves owe their nyctitropic movements to their 
habit of circumnutating — a habit common to all plants, 
and everywhere ready for any beneficial development or 
modification. 

It has been shown in the previous chapters that the 
leaves and cotyledons of all plants are continually moving 
up and down, generally to a slight but sometimes to a con- 
siderable extent, and that they describe either one or several 
ellipses in the course of twenty-four hours ; they arc also 
so far affected by the alternations of day and night that 
they generally, or at least often, move periodically to a 
small extent ; and here we have a basis for the develop- 
ment of the greater nyctitropic movements. That the 
movements of leaves and cotyledons which do not sleep 
come within the class of circumnutating movements can- 
not be doubted, for they are closely similar to those of 
hypocotyls, epicotyls, the stems of mature plants, and of 
various other organs. Now, if we take the simplest case 
of a sleeping leaf, w^e see that it makes a single ellipse in 
the twenty-four hours, which resembles one described by 
a non-sleeping leaf in every respect, except that it is 
much larger. In both cases the course pursued is often 
zigzag. As all non-sleeping leaves are incessantly cir- 
cumnutating, we must conclude that a part at least of 
the upward and downward movement of one that sleeps, 
is due to ordinary circumnutation ; and it seems alto- 
gether gratuitous to rank the remainder of the move- 
ment under a wholly different head. With a multitude 
of climbing plants the ellipses which they describe have 
been greatly increased for another purpose, namely, 
catching hold of a support. With these climbing plants, 
the various circumnutating organs have been so far mod- 
ified in relation to light that, differently from all ordi- 



416 MODIFIED CmCUMNUTATION. Chap. VII. 

nary plants, they do not bend towards it. With sleeping 
plants the rate and amplitude of the movements of the 
leaves have been so far modified in relation to light, 
that they move in a certain direction with the waning 
light of the evening and with the increasing light of the 
morning more rapidly, and to a greater extent, than at 
other hours. 

But the leaves and cotyledons of many non-sleeping 
plants move in a much more complex manner than in 
the cases just alluded to, for they describe two, three, 
or more ellipses in the course of a day. Now, if a plant 
of this kind were converted into one that slept, one side 
of one of the several ellipses which each leaf daily de- 
scribes, would have to be greatly increased in length in 
the evening, until the leaf stood vertically, when it would 
go on circumnutating about the same spot. On the 
following morning, the side of another ellipse would have 
to be similarly increased in length, so as to bring the leaf 
back again into its diurnal position, when it would again 
circumnutate until the evening. If the reader will look, 
for instance, at the diagram (Fig. 142, p. 355), represent- 
ing the nyctitropic movements of the terminal leaflet of 
Trifoliuin subterraneum, remembering that the curved 
broken lines at the top ought to be prolonged much 
higher up, he will see that the great rise in the evening 
and the great fall in the morning together form a large 
ellipse like one of those described during the daytime, 
differing only in size. Or, he may look at the diagram 
(Fig. 103, p. 236) of the 3^ ellipses described in the 
course of 6 h. 35 m. by a leaf of Lnpinus speciosus^ 
which is-one of the species in this genus that does not 
sleep ; and he will see that by merely prolonging upwards 
the line which was already rising late in the evening, and 
bringiug it down again next morning, the diagram would 
represent the movements of a sleeping plant. 



Chap, VII. SUMMARY ON SLEEP OF LEAVES. 4IY 

With those sleeping plants which describe several 
ellipses in the daytime, and which travel in a strongly 
zigzag line, often making in their course minute loops, 
triangles, &c., if as soon as one of the ellipses begins in 
the evening to be greatly increased in size, dots are made 
every 2 or 3 minutes and these are joined, the line then 
described is almost strictly rectilinear, in strong contrast 
with the lines made during the daytime. This was ob- 
served with Desmodium gyrans and Mimosa pudica. 
With this latter plant, moreover, the pinnse converge in 
the evening by a steady movement, whereas during the 
day they are continually converging and diverging to a 
slight extent. In all such cases it was scarcely possible 
to observe the difference in the movement during the 
day and evening, without being convinced that in the 
evening the plant saves the expenditure of force by not 
moving laterally, and that its whole energy is now ex- 
pended in gaining quickly its proper nocturnal position 
by a direct course. In several other cases, for instance, 
when a leaf after describing during the day one or more 
fairly regular ellipses, zigzags much in the evening, it 
appears as if energy was being expended, so that the 
great evening rise or fall might coincide with the period 
of the day proper for this movement. 

The most complex of all the movements performed 
by sleeping plants, is that when leaves or leaflets, after 
describing in the daytime several vertically directed 
ellipses, rotate greatly on their axes in the evening, by 
which twisting movement they occupy a wholly different 
position at night to what they do during the day. For 
instance, the terminal leaflets of Cassia not only move 
vertically downwards in the evening, but twist round, so 
that their lower surfaces face outwards. Such move- 
ments are wholly, or almost wholly, confined to leaflets 
provided with a pulvinus. But this torsion is not a new 



418 MODIFIED CIRCUMNUTATION. Chap. YII. 

kind of movement introduced solely for the purpose of 
sleep; for it has been shown that some leaflets whilst 
describing their ordinary ellipses during the daytime 
rotate slightly, causing their blades to face first to one 
side and then to another. Although w^e can see how the 
slight periodical movements of leaves in a vertical plane 
could be easily converted into the greater yet simple 
nyctitropic movements, we do not at present know by 
what graduated steps the more complex movements, 
effected by the torsion of the pulvini, have been ac- 
quired. A probable explanation could be given in each 
case only after a close investigation of the movements in 
all the allied forms. 

From the facts and considerations now advanced we 
may conclude that nyctitropism, or the sleep of leaves 
and cotyledons, is merely a modification of their ordi- 
nary circumnutating movement, regulated in its period 
and amplitude by the alternations of light and darkness. 
The object gained is the protection of the upper surfaces 
of the leaves from radiation at night, often combined 
with the mutual protection of the several parts by their 
close approximation. In such cases as those of the leaf- 
lets of Cassia — of the terminal leaflets of Melilotus — of 
all the leaflets of Arachis, Marsilea, &c. — we have ordi- 
nary circumnutation modified to the extreme extent 
known to us in any of the several great classes of modi- 
fied circumnutation. On this view of the origin of nyc- 
titropism we can understand how it is that a few plants, 
widely distributed throughout the Vascular series, have 
been able to acquire the habit of placing the blades of 
their leaves vertically at night, that is, of sleeping, — a 
fact otherwise inexplicable. 

The leaves of some plants move during the day in a 
manner, which has improperly been called diurnal sleep ; 



Chap. VII. SUMMARY ON SLEEP OF LEAVES. 419 

for when the sun shines brightly on them, they direct 
their edges towards it. To such cases we shall recur in 
the following chapter on Heliotropism. It has been 
shown that the leaflets of one form of Porlieria liygro- 
metrica keep closed during the day, as long as the plant 
is scantily supplied with water, in the same manner as 
when asleep ; and this apparently serves to check evapo- 
ration. There is only one other analogous case known 
to us, namely, that of certain Gramineae, which fold in- 
wards the sides of their narrow leaves, when these are 
exposed to the sun and to a dry atmosphere, as described 
by Duval-Jouve.* We have also observed the same phe- 
nomenon in Ely7niis arenareus. 

There is another movement, which since the time of 
Linnaeus has generally been called sleep, namely, that of 
the petals of the many flowers which close at night. 
These movements have been ably investigated by Pfeffer, 
who has shown (as was first observed by Hofmeister) that 
they are caused or regulated more by temperature than 
by the alternations of light and darkness. Although 
they cannot fail to protect the organs of reproduction 
from radiation at night, this does not seem to be their 
chief function, but rather the protection of the organs 
from cold winds, and especially from rain, during the 
day. The latter seems probable, as Kerner \ has shown 
that a widely different kind of movement, namely, 
the bending down of the upper part of the peduncle, 
serves in many cases the same end. The closure of 
the flowers will also exclude nocturnal insects which 
may be ill-adapted for their fertilisation, and the well- 
adapted kinds at periods when the temperature is not 
favourable for fertilisation. Whether these movements 



*'Annal. des Sc. Nat. (Bot.),' f ' Die Schutzmittel des Pollens, 
1875, torn. i. pp. 330-329. 1873, pp. 30-39. 



420 STRUCTURE OF Chap. VII. 

of the petals consist, as is probable, of modified circum- 
nutation we do not know. 

Emlryology of Leaves. — A few facts have been inci- 
dentally given in this chapter on what may be called the 
embryology of leaves. With most plants the first leaf 
which is developed after the cotyledons, resembles closely 
the leaves produced by the mature plant, but this is not 
always the case. The first leaves produced by some spe- 
cies of Drosera, for instance by D. Capensis^ differ widely 
in shape from those borne by the mature plant, and re- 
semble closely the leaves of D. rotundifolia, as was shown 
to us by Prof. Williamson of Manchester. The first true 
leaf of the gorse, or Ulex, is not narrow and spinose like 
the older leaves. On the other hand, with many Legu- 
minous plants, for instance, Cassia, Acacia lophantha^ &c., 
the first leaf has essentially the same character as the 
older leaves, excepting that it bears fewer leaflets. In 
Trifolium the first leaf generally bears only a single 
leaflet instead of three, and this differs somewhat in shape 
from the corresponding leaflet on the older leaves. Now, 
with Trifolium Pannonicum the first true leaf on some 
seedlings was nnifoliate, and on others completely tri- 
foliate ; and between these two extreme states there were 
all sorts of gradations, some seedlings bearing a single 
leaflet more or less deeply notched on one or both sides, 
and some bearing a single additional and perfect lateral 
leaflet. Here, then, we have the rare opportunity of see- 
ing a structure proper to a more advanced age, in the act 
of gradually encroaching on and replacing an earlier or 
embryological condition. 

The genus Melilotus is closely allied to Trifolium, 
and the first leaf bears only a single leaflet, which at 
night rotates on its axis so as to present one lateral edge 
to the zenith. Hence it sleeps like the terminal leaflet 



Chap. VII. FIRST-FORMED LEAVES. 421 

of a mature plant, as was observed in 15 species, and 
wholly unlike the corresponding leaflet of Trifolium, 
which simply bends upwards. It is therefore a curious 
fact that in one of these 15 species, viz., M. Taurica 
(and in a lesser degree in two others), leaves arising from 
young shoots, produced on plants which had been cut 
down and kept in pots during the winter in the green- 
house, slept like the leaves of a Trifolium, whilst the 
leaves on the fully-grown branches on these same plants 
afterwards slept normally like those of a Melilotus. If 
young shoots rising from the ground may be considered 
as new individuals, partaking to a certain extent of the 
nature of seedlings, then the peculiar manner in v/hich 
their leaves slept may be considered as an embryological 
habit, probably the result of Melilotus being descended 
from some form which slept like a Trifolium. This 
view is partially supported by the leaves on old and 
young branches of another species, M. Messanensis (not 
included in the above 15 species), always sleeping like 
those of a Trifolium. 

The first true leaf of Mimosa albida consists of a 
simple petiole, often bearing three pairs of leaflets, all of 
which are of nearly equal size and of the same shape : 
the second leaf differs widely from the first, and resembles 
that on a mature plant (see Fig. 159, p. 391), for it con- 
sists of two pinnae, each of which bears two pairs of 
leaflets, of which the inner basal one is very small. But 
at the base of each pinnae there is a pair of minute points, 
evidently rudiments of leaflets, for they are of unequal 
sizes, like the two succeeding leaflets. These rudiments 
are in one sense embryological, for they exist only during 
the youth of the leaf, falling off and disappearing as soon 
as it is fully grown. 

With Desmodium gyrans the two lateral leaflets are 
very much smaller than the corresponding leaflets -in 



422 MODIFIED CIRCUMNUTATION. Chap. VII. 

most of the species in this large genus; they vary also 
in position and size ; one or both are sometimes absent ; 
and they do not sleep like the fully-developed leaflets. 
They may therefore be considered as almost rudimentary ; 
and in accordance with the general principles of em- 
bryology, they ought to be more constantly and fully 
developed on very young than on old plants. But this 
is not the case, for they were quite absent on some young 
seedlings, and did not appear until from 10 to 20 leaves 
had been formed. This fact leads to the suspicion that 
D. gyrmis is descended through a unifoliate form (of 
which some exist) from a trifoliate species ; and that the 
little lateral leaflets reappear through reversion. How- 
ever this may be, the interesting fact of the pulvini or 
organs of movement of these little leaflets, not having 
been reduced nearly so much as their blades — taking the 
large terminal leaflet as the standard of comparison — 
gives us probably the proximate cause of their extraor- 
dinary power of gyration. 



p 



CHAPTER VIII. 

Modified Circumnutatton : Movements excited by Light. 

Distinction between heliotropism and the eflfects of light on the perio- 
dicity of the movements of leaves — Heliotropic movements of Beta, 
Solanum, Zea, and Avena — Heliotropic movements towards an 
obscure light in Apios, Brassica, Phalaris, Tropseolum, and Cassia 
— Apheliotropic movements of tendrils of Bignonia — Of flower- 
peduncles of Cyclamen— Burying of the pods — Heliotropism and 
apheliotropism modified forms of circumnutation — Steps by which 
one movement is converted into the other — Transversal heliotrop- 
isums or diaheliotropism, influenced by epinasty, the weight of 
the part and apogeotropism — Apogeotropism overcome during the 
middle of the day by diaheliotropism — Effects of the weight of 
the blades of cotyledons — So-called diurnal sleep — Chlorophyll 
injured by intense light— Movements to avoid intense light. 

Sachs first clearly pointed out the important differ- 
ence between the action of light in modifying the pe- 
riodic movements of leaves, and in causing them to bend 
towards its source.* The latter, or heliotropic move- 
ments are determined by the direction of the light, whilst 
periodic movements are affected by changes in its inten- 
sity and not by its direction. The periodicity of the 
circumnutating movement often continues for some time 
in darkness, as we have seen in the last chapter ; whilst 
heliotropic bending ceases very quickly when the light 
fails. Nevertheless, plants which have ceased through 
long - continued darkness to move periodically, if re- 
exposed to the light are still, according to Sachs, he- 
liotropic. 

Apheliotropism, or, as usually designated, negative 



* 'Physiologic Veg.' (French Translation), 1868, pp. 42, 517, &c. 
28 423 



424 MODIFIED CIRCUMNUTATION. Chap. VIII. 

heliotropism, implies that a plant, when unequally illu- 
minated on the two sides, bends from the light, instead 
of, as in the last sub-class of cases, towards it ; but aphe- 
liotropism is comparatively rare, at least in a well-marked 
degree. There is a third and large sub-i^lass of cases, 
namely, those of " Transversal-Heliotropismus " of Frank, 
which we will here call diaheliotropism. Parts of plants, 
under this influence, place themselves more or less trans- 
versely to the direction whence the light proceeds, and 
are thus fully illuminated. There is a fourth sub-class, 
as far as the final cause of the movement is concerned ; 
for the leaves of some plants when exposed to an intense 
and injurious amount of light direct themselves, by rising 
or sinking or twisting, so as to be less intensely illumi- 
nated. Such movements have sometimes been called 
diurnal sleep. If thought advisable, they might be called 
paraheliotropic, and this term would correspond with 
our other terms. 

It will be shown in the present chapter that all the * 
movements included in these four sub-classes, consist of 
modified circumnutation. We do not pretend to say 
that if a part of a plant, whilst still growing, did not 
circumnutate — though such a supposition is most im- 
probable — it could not bend towards the light ; but, as a 
matter of fact, heliotropism seems always to consist of 
modified circumnutation. Any kind of movement in 
relation to light will obviously be much facilitated by 
each part circumnutating or bending successively in all 
directions, so that an already existing movement has only 
to be increased in some one direction, and to be lessened 
or stopped in the other directions, in order that it should 
become heliotropic, apheliotropic, &c., as the case may 
be. In the next chapter some observations on the sen- 
sitiveness of plants to light, their rate of bending towards 
it, and the accuracy with which they point towards its 



Chap. VIII. MOVEMENTS EXCITED BY LIGHT, 425 

source, &c., will be given. Afterwards it will be shown 
— and this seems to us a point of much interest — that 
sensitiveness to light is sometimes confined to a small 
part of the plant ; and that this part when stimulated by 
light, transmits an influence to distant parts, exciting 
them to bend. 

Heliotropism. — When a plant which is strongly helio- 
tropic (and species differ much in this respect) is exposed 
to a bright lateral light, it bends quickly towards it, and 
the course pursued by the stem is quite or nearly straight. 
But if the light is much dimmed, or occasionally inter- 
rupted, or admitted in only a slightly oblique direction, 
the course pursued is more or less zigzag; and as we have 
seen and shall again see, such zigzag movement results 
from the elongation or drawing out of the ellipses, loops, 
&c., which the plant would have described, if it had 
been illuminated from above. On several occasions we 
were much struck with this fact, whilst observing the 
circumnutation of highly sensitive seedlings, which were 
unintentionally iUuminated rather obliquely, or only at 
successive intervals of time. 

For instance, two young seedlings of Beta vulgaris were 
placed in the middle of a room with north-east windows, and 
were kept covered up, except during each observation which 
lasted for only a minute or two ; but the result was that their 
hypocotyls bowed themselves to the side, whence some light 
occasionally entered, in lines which were only shghtly zigzag. 
Although not a single ellipse was even approximately formed, 
we inferred from the zigzag lines — and, as it proved, correctly — 
that their hypocotyls were circumnutating, for on the following 
day these same seedlings were placed in a completely darkened 
room, and were observed each time by the aid of a small wax 
taper held almost directly above them, and their movements 
were traced on a horizontal glass above ; and now their hypo- 
cotyls clearly circumnutated (Fig. 168, and Fig. 39, formerly 
given, p. 52) ; yet they moved a short distance towards the side 



426 



MODIFIED CIRCUMNUTATION. Chap. VIII. 



Fig. 168. 




where the taper was held up. If we look at these diagrams, 
and suppose that the taper had been held more on one side, 
and that the hypocotyls, still circumnutating, had bent them- 
selves within the same time much more 
towards the light, long zigzag lines 
would obviously have been the result. 

Again, two seedlings of Solarium 
lycopersicum were illuminated from 
above, but accidentally a little more 
light entered on one than on any other 
side, and their hypocotyls became 
slightly bowed towards the brighter 
side ; they moved in a zigzag line and 
described in their course two little tri- 
angles, as seen in Fig. 37 (p. 51), and 
in another tracing not given. The 
sheath-like cotyledons of Zea mays be- 
haved, under nearly similar circum- 

l^^ T-^^ Direction of stances, in a nearly similar manner, as 
the lighted taper by ' -^ ' 

described in our first chapter (p. 65), 

for they bowed themselves during the 
whole day towards one side, making, 
however, in their course some conspicu- 
ous flexures. Before we knew how 
greatly ordinary circumnutation was modified by a lateral lio-ht, 
some seedling oats, with rather old and therefore not highly 
sensitive cotyledons, were placed in front of a north-east win- 
dow, towards which they bent all day in a strongly zigzag 
course. On the following day they continued to bend in the 
same direction (Fig. 169), but zigzagged much less. The sky, 
however, became between 12.40 and 2.35 p.m. overcast with 
extraordinarily dark thunder-clouds, and it was interesting to 
note how plainly the cotyledons circumnutated during this 
interval. 

The foregoing observations are of some value, from having 
been made when we were not attending to heliotropism ; and 
they led us to experiment on several kinds of seedlings, by ex- 
posing them to a dim lateral light, so as to observe the gra- 
dations between ordinary circumnutation and heliotropism. 



Beta vulgaris : circumnu- 
tation of hypocotyl, de- 
flected by the light be- 
ing slightly lateral, 
traced on a horizontal 
glass from 8.30 a.m. to 



which it was illumi- 
nated, shown by a line 
joining the first and 
penultimate dots. Fig- 
ure reduced co one-third 
of the original scale. 



Chap. VIII. 



HELIOTROPISM. 



427 



Fig. 169. 



Seedlings in pots were placed in front of, and about a yard from, 
a north-east window ; on eacii side and over the pots black boards 
were placed ; in the rear the pots were open to the diffused light- 
of the room, which had a second north-east and a north-west 
window. By hanging up one or more blinds before the window 
where the seedlings stood, it was easy to dim the light, so that 
very little more entered on this side than on the opposite one, 
which received the diffused light of the room. Late in the 
evening the blinds were successively re- 
moved, and as the plants had been sub- 
jected during the day to a very obscure 
light, they continued to bend towards 
the window later in the evening than 
would otherwise have occurred. Most 
of the seedlings were selected because 
they were known to be highly sensitive 
to light, and some because they were 
but little sensitive, or had become so 
from having grown old. The move- 
ments were traced in the usual manner 
on a horizontal glass cover ; a fine glass 
filament with little triangles of paper 
having been cemented in an upright 
position to the hypocotyls. Whenever 
the stem or hypocotyl became much 
bowed towards the light, the latter part 
of its course had to be traced on a verti- 
cal glass, parallel to the window, and 
at right angles to the horizontal glass 
cover. 

Apios graveolens. — The hypocotyl 
bends in a few hours rectangularly to- 
wards a bright lateral light. In order 
to ascertain how straight a course it 
would pursue when fairly well illumi- 
nated on one side, seedlings were first 
placed before a south-west window on 
a cloudy and rainy morning; and the movement of two hypo- 
cotyls were traced for 3 h., during which time they became 




8°a.in: 



Avena saliva : heliotropic 
movement and circum- 
nutation of sheath -like 
cotyledon (li inch in 
height) traced on hori- 
zontal glass from 8 a.m. 
to 10.25 P.M. Oct. 16th. 



428 MODIFIED CIRCUMNUTATION. Chap. VIII. 

greatly bowed towards the light. One of these tracings is 
given in Fig. 170, and the course may be seen to be al- 
most straight. But the amount of light on this occasion was 
superfluous, for two seedlings were placed before a north-east 
window, protected by an ordinary linen 
^ o and two muslin blinds, yet their hypo- 
Mr^ cotyls moved towards this rather dim 
'^ § light in only slightly zigzag lines ; but 
% g after 4 p.m., as the light waned, the 
I 2 lines became distinctly zigzag. One of 
'o.^ these seedlings, moreover, described in 
^ the afternoon an ellipse of considerable 

rg^ size, with its longer axis directed to- 
g'-i wards the window. 
^ ft We now determined that the light 

;S^ should be made dim enough, so we be- 
'S J g^^ by exposing several seedlings be- 
pa o fore a north-east window, protected by 
^;i| one linen blind, three muslin blinds, 
.2 S and a towel. But so little light entered 
o ^ that a pencil cast no perceptible shadow 



-* 



< 



^ o on a white card, and the hypocotyls did 

'Fjqo not bend at all towards the window. 

8 I During this time, from 8.15 to 10.50 

^"^ A.M., the hypocotyls zigzagged or cir- 

<4_, J cumnutated near the same spot, as may 

■g-^ be seen at A, in Fig. 171. The towel, 

g fl therefore, was removed at 10.50 a.m., 

^•^ and replaced by two muslin blinds, and 

B^ ^ now the light passed through one ordi- 

'Ert « nary linen and four muslin blinds. 

^^^ When a pencil was held upright on a 

^ |-5i card close to the seedlings, it cast a 

'^^o shadow (pointing from the window) 

|S o which could only just be distinguished. 

1*;^.^ Yet this very slight excess of light on 

g I "^ one side sufficed to cause the hypo- 

§3 o cotyls of all the seedlings immediately 

3 to begin bending in zigzag lines to- 



Chap. VIII. 



HBLIOTROPISM. 



429 



wards the window. The course of one is shown at A (Fig. 
171): after moving towards the window from 10.50 a.m. to 
13.48 P.M. it bent from the window, and then returned in a 

Fig. 171, 




is:M\ 



fO:50!'m 




lO^'SO^a.m. 



Apios graveolens : heliotropic movement and circumnutation of the 
hypocotyls of two seedlings towards a dim lateral light, traced on 
a horizontal glass during the day. The broken lines show their 
return nocturnal courses. Height of hypocotyl of A "5, and of B 
•55 inch. Figure reduced to one-half of original scale. 



nearly parallel line; that is, it almost completed between 12.48 
and 3 p.m. a narrow ellipse. Late in the evening, as the light 



430 MODIFIED CIRCUMNUTATION. Chap. VIII. 

waned, the hypocotyl ceased to bend towards the window, and 
circumnutated on a small scale round the same spot; during 
the night it moved considerably backwards, that is, became 
more upright, through the action of apogeotropism. At B, we 
have a tracing of the movements of another seedling from the 
hour (10.50 A.M.) when the towel was removed; and it is in all 
essential respects similar to the previous one. In these two 
cases there could be no doubt that the ordinary circumnutating 
movement of the hypocotyl was modified and rendered helio- 
tropic. 

Brassica oleracece. — The hypocotyl of the cabbage, when not 
disturbed by a lateral light, circumnutates in a complicated 
manner over nearly the same space, and a figure formerly given 
is here reproduced (Fig. 172). If the hypocotyl is exposed to 
a moderately strong lateral light it moves quickly towards this 
side, travelling in a straight, or nearly straight, line. But when 
the lateral light is very dim its course is extremely tortuous, and 

Fig. 172. 




Brassica oleracea : ordinary circnmnntatiug movement of the hypo- 
cotyl of a seedling plant. 

evidently consists of modified circumnutation. Seedlings were 
placed before a north-east window, protected by a linen and 
muslin blind and by a towel. The sky was cloudy, and when- 



Chap. VIII. 



HELIOTROPISM. 



431 



ever the clouds grew a little lighter an additional muslin blind 
was temporarily suspended. The light from the window was 
thus so much obscured that, judging by the unassisted eye, the 

Fig. 173. 




Brassica oUracea : heliotropic movement and circumnutation of a hypo- 
cotyl towards a very dim lateral light, traced during 11 houi-s, on 
a horizontal glass in the morning, and on a vertical glass in the 
evening. Figure reduced to one-third of the original scale. 



432 MODIFIED CIRCUMNUTATION. Chap. VIII. 

seedlings appeared to receive more light from the interior 
of the room than from the window ; but this was not really 
the case, as was shown by a very faint shadow cast by a pencil 
on a card. Nevertheless, this extremely small excess of light 
on one side caused the hypocotyls, which in the morning had 
stood upright, to bend at right angles towards the window, 
so that in the evening (after 4.23 p.m.) their course had to be 
traced on a vertical glass parallel to the window. It should be 
stated that at 3.30 p.m., by which time the sky had become 
darker, the towel was removed and replaced by an additional 
muslin blind, w^hich itself was removed at 4 p.m., the other two 
blinds being left suspended. In Fig 173 the course pursued, 
between 8.09 a.m. and 7.10 p.m., by one of the hypocotyls thus 
exposed is shown. It may be observed that during the first 
16 m. the hypocotyl moved obliquely from the light, and this, 
no doubt, was due to its then circumnutating in this direction. 
Similar cases were repeatedly observed, and a dim light rarely 
or never produced any effect until from a quarter to three-quar- 
ters of an hour had elapsed. After 5.15 p.m., by which time 
the light had become obscure, the hypocotyl began to circumnu- 
tate about the same spot. The contrast between the two figures 
(172 and 173) would have been more striking, if they had been 
originally drawn on the same scale, and had been equally 
reduced. But the movements shown in Fig. 172 were at first 
more magnified, and have been reduced to only one-half of the 
original scale; whereas those in Fig. 173 were at first less mag- 
nified, and have been reduced to a one-third scale. A tracing 
made at the same time with the last of the movements of a 
second hypocotyl, presented a closely analogous appearance; 
but it did not bend quite so much towards the light, and it 
circumnutated rather more plainly. 

Phalaris Oanariensis.— The sheath-like cotyledons of this 
monocotyledonous plant were selected for trial, because they 
are very sensitive to light and circumuutate well, as formerly 
shown (see Fig. 49, p. 64). Although we felt no doubt about 
the result, some seedlings were first placed before a south-west 
window on a moderately bright morning, and the movements 
of one were traced. As is so common, it moved for the first 
45 m. in a zigzag line; it then felt the full influence of the 



Chap. VIII. 



HELIOTROPISM. 



433 



S^.m 



light, and travelled towards it for the next 2 h. 30 m. in an 
almost straight line. The tracing has not been given, as it was 
almost identical with that of Apios under similar circum- 
stances (Fig. 170). By noon it had bowed itself to its full 
extent ; it then circum- 

nutated about the Fig- l^^. 

same spot and de- 
scribed two ellipses; 
by 5 P.M. it had re- 
treated considerably 
from the light, through 
the action of apogeot- 
ropism. After some 
preliminary trials for 
ascertaining the right 
degree of obscurity, 
some seedlings were 
placed (Sept. 16th) be- 
fore a north-east win- 
dow, and light was 
admitted through an 
ordinary linen and 
three muslin blinds. 
A pencil held close by 
the pot now cast a 
very faint shadow on 
a white card, pointing 
from the window. In 
the evening, at 4.30, 
and again at 6 p.m., 
some of the blinds 
were removed. In Fig. 
174 we see the course 
pursued under these 
circumstances by a 
rather old and not very sensitive cotyledon, 1-9 inch in height, 
which became much bowed, but was never rectangularly bent 
towards the light. From 11 a.m., when the sky became rather 
duller, until 6.30 p.m., the zigzagging was conspicuous, and 




8ll5^a.m.Sepm*l^ 



Phalaris Canariensis : heliotropic movement 
and circumnutation of a rather old coty- 
ledon, towards a dull lateral light, traced 
on a horizontal glass from 8.15 a.m. Sejit. 
16th to 7.45 A.m. 17th. Figure reduced 
to one-tliird of original scale. 



434 MODIFIED CIRCUMNUTATION. Chap. VIII. 

evidently consisted of drawn-out ellipses. After 6.30 p.m. and 
during the night, it retreated in a crooked line from the win- 
dow. Another and younger seedling moved during the same 
time much more quickly and to a much greater distance, in an 
only slightly zigzag line towards the light; by 11 a.m. it was 
bent almost rectangularly in this direction, and now circumnu- 
tated about the same place. 

TropoBolum majus. — Some very young seedlings, bearing only 
two leaves, and therefore not as yet arrived at the climbing 
stage of growth, were first tried before a north-east window 



Fig. 175, 



i0'40'j9.m. 




7'48'a.m< 



Tropseolum majus: lieliotropic movement and circumnutation of the 
epicotyl of a young seedling towards a dull lateral light, traced on 
a horizontal glass from 7.48 A.M. to 10.40 P.M. Figure reduced to 
one-half of the original scale. 

without any blind. The epicotyls bowed themselves towards 
the light so rapidly that in little more than 3 h. their tips 
pointed rectangularly towards it. The lines traced were either 
nearly straight or slightly zigzag; and in this latter case we 
see that a trace of circumnutation was retained even under the 



Chap. VIII. HELIOTROPISM. 435 

influence of a moderately bright light. Twice whilst these 
epieotyls were bending towards the window, dots were made 
every 5 or 6 minutes, in order to detect any trace of lateral 
movement, but there was hardly any; and the lines formed by 
their junction were nearly straight, or only very slightly zigzag, 
as in the other parts of the figures. After the epieotyls had 
bowed themselves to tlie full extent towards the light, ellipses 
of considerable size were described in the usual manner. 

After having seen how the epieotyls moved towards a mod- 
erately bright light, seedlings were placed at 7.48 a.m. (Sept. 
7th) before a north-east window, covered by a towel, and shortly 
afterwards by an ordinary linen blind, but the epieotyls still 
moved towards the window. At 9. 30 a.m. two additional muslin 
blinds were suspended, so that the seedlings received very little 
more light from the window than from the interior of the room. 
The sky varied in brightness, and the seedlings occasionally 
received for a short time less light from the window than from 
the opposite side (as ascertained by the shadow cast), and then 
one of the blinds was temporarily removed. In the evening 
the blinds were taken away, one by one. The course pursued 
by an epicotyl under these circumstances is shown in Fig, 175. 
During the whole day, until 6,45 p.m., it plainly bowed itself 
towards the light ; and the tip moved over a considerable space. 
After 6.45 p.m. it moved backwards, or from the window^, till 
10,40 p,M,, when the last dot was made. Here, then, we have 
a distinct heliotropic movement, effected by means of six 
elongated figures (which if dots had been made every few 
minutes would have been more or less elliptic) directed towards 
the light, with the apex of each successive ellipse nearer to the 
window than the previous one. Now, if the light had been 
only a little brighter, the epicotyl would have bowed itself 
more to the light, as we may safely conclude from the previous 
trials ; there would also have been less lateral movement, and 
the ellipses or other figures would have been drawn out into a 
strongly marked zigzag line, with probably one or two small 
loops still formed. If the light had been much brighter, we 
should have had a slightly zigzag line, or one quite straight, 
for there would have been more movement in the direction of 
the light, and much less from side to side. 



436 



MODIFIED CIRCUMNUTATION. Chap. VIII. 



Sachs states that the older internodes of this TropEeolum 
are apheliotropic; we therefore placed a plant, llf inches high, 
in a box, blackened within, but 
open on one side in front of a 
north-east window without any 
blind. A filament was fixed to the 
third internode from the summit 
on one plant, and to the fourth in- 
ternode of another. These inter- 
nodes were either not old enough, 
or the light was not sufficiently 
bright, to induce apheliotropism, 
for both plants bent slowly to- 
wards, instead of from the window 
during four days. The course, 
during two days of the first-men- 
tioned internode, is given in Fig. 
176 ; and we see that it either cir- 
cumnutated on a small scale, or 
travelled in a zigzag line towards 
the light. We have thought this 
case of feeble heliotropism in one 
of the older internodes of a plant, 
which, whilst young, is so extreme- 
ly sensitive to light, worth giving. 
Cassia tora. — The cotyledons 
of this plant are extremely sensi- 
tive to light, whilst the hypocotyls 
are much less sensitive than those 
of most other seedlings, as we had 
often observed with surprise. It 
seemed therefore worth while to 
trace their movements. They were exposed to a lateral light 
before a north-east window, which was at first covered merely 
by a muslin blind, but as the sky grew brighter about 11 a.m., 
an additional linen blind was suspended. After 4 p.m. one 
blind and then the other was removed. The seedlings were pro- 
tected on each side and above, but were open to the diffused 
light of the room in the rear. Upright filaments were fixed to 




Tropseolum majus : lieliotropic 
movement and circumnuta- 
tion of an old internode to- 
wards a lateral light, traced 
on a horizontal glass from 8 
A.M. ISTov. 2nd to 10.20 a.m. 
Nov. 4th. Broken lines 
show the nocturnal course. 



Chap. YIII. 



HELIOTROPISM. 



437 



the hypocotyls of two seedlings, which stood vertically in the 
morning. The accompanying figure (Fig. 177) shows the course 
pursued by one of them during two days; but it should be par- 
ticularly noticed that during the second day the seedlings were 
kept in darkness, and they then circumnutated round nearly 
the same small space. On 



the first day (Oct. 7th) the 
hypocotyl moved from 8 
A.M. to 13.23 P.M., toward 
the light in a zigzag line, 
tlien tui-ned abruptly to 
the left and afterwards 
described a small ellipse. 
Another irregular ellipse 
was completed between 3 
P.M. and about 5.30 p.m., 
the hypocotyl still bending 
towards the light. The 
hypocotyl was straight and 
upright in the morning, but 
by 6 p.m. its upper half was 
bowed towards the light, 
so that the chord of the arc 
thus formed stood at an 
angle of 20° with the per- 
pendicular. After 6 p.m. 
its course was reversed 
through the action of apo- 
geotropism, and it con- 
tinued to bend from the 
window during the night, 
as shown by the broken 
line. On the next day it 
was kept in the dark (ex- 
cepting when each observa- 
tion was made by the aid 
of a taper), and the course 
followed from 7 a.m. on the 
8th to 7.45 a.m. on the 9th 




6jo.m7 



bh 



'0''lC>]9:m7i^ 



/ V 

8hm.7*^ ^ 

Cassia torn : heliotropic movement and 
circumnutation of a hypocotyl (1^ 
inch in height ) traced on a horizon- 
tal glass from 8 a.m. to 10.10 p.m. Oct. 
7th. Also its circumnutation in dark- 
ness from 7 a.m. Oct. 8th to 7.45 A.M. 
Oct. Oth. 



438 



MODIFIED CmCUMNUTATION. Chap. VIII. 



Fig. 178. 



Bignonia capreolata : aphe- 
liotropic moveBient of a 
tendril, traced on a hori- 
zontal glass from 6.45 
A.M. July 19th to 10 a.m. 
20th. Movements as 
originally traced, little 
magnified, here reduced 
to two-thirds of the origi- 
nal scale. 



is here likewise shown. The differ- 
ence between the two parts of the 
figure (177), namely, that described 
during the daytime on the 7th, when 
exposed to a rather dim lateral light, 
and that on the 8th in darkness, is 
striking. The di£Eerence consists in 
the lines during the first day having 
been drawn out in the direction of the 
light. The movements of the other 
seedling, traced under the same cir- 
cumstances, were closely similar. 

ApJieliotropism. — We succeeded in 
observing only two cases of aphelio- 
tropism, for these are somewhat rare ; 
and the movements are generally so 
slow that they would have been very 
troublesome to trace. 

Bignonia capreolata. — No organ of 
any plant, as far as we have seen, 
bends away so quickly from the light 
as do the tendrils of this Bignonia. 
They are also remarkable from cir- 
cumnutating much less regularly than 
most other tendrils, often remaining 
stationary; they depend on apheliot- 
ropism for coming into contact with 
the trunks of trees.* The stem of a 
young plant was tied to a stick at the 
base of a pair of fine tendrils, which 
projected almost vertically upwards; 
and it was placed in front of a north- 
east window, being protected on all 
other sides from the light. The first 
dot was made at 6.45 a.m., and by 
7.35 A.M. both tendrils felt the full 
influence of the light, for they moved 



The Movements and Habits of Climbing Plants,' 1875, p. 97. 



Chap. VIII. APHELIOTROPISM. 439 

straight away from it until 9.20 a.m., wlien they circumnu- 
tated for a time, still moving, but only a little, from the light 
(see Fig. IvS of the left-hand tendril). After 3 p.m. they 
again moved rapidly away from the light in zigzag lines. By 
a late hour in the evening both had moved so far, that they 
pointed in a direct Ime from the light. During the night they 
returned a little in a nearly opposite direction. On the follow- 
ing morning they again moved from the light and converged, 
so that by the evening they had become interlocked, still point- 
ing from the light. The right-hand tendril, whilst converging, 
zigzagged much more than the one figured. Both tracings 
showed that the apheliotropic movement was a modified form 
of circumnutation. 

Cyclamen Persicum. — Whilst this plant is in flower the 
peduncles stand upright, but their uppermost part is hooked so 
that the flower itself hangs downwards. As soon as the pods 
begin to swell, the peduncles increase much in length and slowly 
curve downwards, but the short, upper, hooked part straightens 
itself. Ultimately the pods reach the ground, and if this is 
covered with moss or dead leaves, they bury themselves. We 
have often seen saucer-like depressions formed by the pods in 
damp sand or sawdust; and one pod (-3 of inch in diameter) 
buried itself in sawdust for three-quarters of its length.* We 
shall have occasion hereafter to consider the object gained by 
this burying process. The peduncles can change the direction 
of their curvature, for if a pot, with plants having their pedun- 
cles already bowed downwards, be placed horizontally, they 
slowly bend at right angles to their former direction towards 
the centre of the earth. We therefore at flrst attributed the 
movement to geotropism; but a pot which had lain horizon- 
tally with the pods all pointing to the ground, was reversed, 
being still kept horizontal, so that the pods now pointed 
directly upwards; it was then placed in a dark cupboard, but 
the pods still pointed upwards after four days and nights. The 
pot, in the same position, was next brought back into the light, 



■■•■ The peduncles of several other den,' Canto, iii. p. 126), the pods 

species of Cyclamen twist thorn- forcibly penetrate the earth. See 

selves into a spire, and according also Grenicr and Godron, 'Flore 

to Erasmus Darwin (' Botanic Gar- de France,' tom. ii. p. 459. 

29 



440 MODIFIED CmCUMNUTATION. Chap. VHP. 

and after two days there was some bending downwards of the 
peduncles, and on the fourth day two of them pointed to the 
centre of the earth, as did the others after an additional day 
or two. Another plant, in a pot which had always stood 
upright, was left in the dark cupboard for six days ; it bore 3 
peduncles, and only one became within this time at all bowed 
downwards, and that doubtfully. The w^eight, therefore, of the 
pods is not the cause of the bending down. This pot was then 
brought back into the light, and after three days the peduncles 
were considerably bowed downwards. We are thus led to infer 
that the downward curvature is due to apheliotropism ; though 
more trials ought to have been made. 

Fig. 179. 





Cyclamen Persicum: downward apbeliotropie movement of a flower- 
peduncle, greatly magnified (about 47 times?), traced on a hori- 
zontal glass from 1 P.M. Feb. 18th to 8 a.m. 21st. 

In order to observe the nature of this movement, a peduncle 
bearing a large pod which had reached and rested on the 
ground, was lifted a little up and secured to a stick. A filament 
was fixed across the pod with a mark beneath, and its move- 
ment, greatly magnified, was traced on a horizontal glass dur- 
ing 67 h. The plant was illuminated during the day from 
above. A copy of the tracing is given (Fig. 179) ; and there can 



Chap. VIIL APHELIOTROPISM. 441 

be no doubt that the descending movement is one of modified 
circumnutation, but on an extremely small scale. The observa- 
tion was repeated on another pod, which had partially buried 
itself in sawdust, and which was lifted up a quarter of an inch 
above the surface; it described three very small circles in 24 h. 
Considering the great length and thinness of the peduncles 
and the lightness of the pods, we may conclude that they 
would not be able to excavate saucer-like depressions in sand 
or sawdust, or bury themselves in moss, &c., unless they were 
aided by their continued rocking or circumnutating move- 
ment. 

Relation ietween Circumnutation and Heliotropism,. 
— Any one who will look at the foregoing diagrams, 
showing the movements of the stems of various plants 
towards a lateral and more or less dimmed light, will be 
forced to admit that ordinary circumnutation and heli- 
otropism graduate into one another. When a plant is 
exposed to a dim lateral light and continues during the 
whole day bending towards it, receding late in the even- 
ing, the movement unquestionably is one of heliotropism. 
Now, in the case of Tropeeolum (Fig. 175) the stem or 
epicotyl obviously circumnutated during the whole day, 
and yet it continued at the same time to move helio- 
tropically ; this latter movement being effected by the 
apex of each successive elongated figure or ellipse stand- 
ing nearer to the light than the previous one. In the 
case of Cassia (Fig. 177) the comparison of the move- 
ment of the hypocoty], when exposed to a dim lateral 
light and to darkness, is very instructive ; as is that be- 
tween the ordinary circumnutating movement of a seed- 
ling Brassica (Figs. 172, 173), or that of Phalaris (Figs. 
49, 174), and their heliotropic movement towards a win- 
dow protected ,by blinds. In both these cases and in 
many others, it was interesting to notice how gradually 
the stems began to circumnutate as the light waned in 



442 CIRCUMNUTATION Chap. YIII. 

the evening. We have therefore many kinds of grada- 
tions from a movement towards the light, which must be 
considered as one of circumnutation very slightly modi- 
fied and still consisting of ellipses or circles, — though a 
movement more or less strongly zigzag, with loops or 
ellipses occasionally formed, — to a nearly straight, or even 
quite straight, heliotropic course. 

A plant, when exposed to a lateral light, though this 
may be bright, commonly moves at first in a zigzag line, 
or even directly from the light ; and this no doubt is 
due to its circumnutating at the time in a direction 
either opposite to the source of the liglit, or more or less 
transversely to it. As soon, however, as the direction of 
the circumnutating movement nearly coincides with that 
of the entering light, the plant bends in a straight course 
towards the light, if this is bright. The course appears 
to be rendered more and more rapid and rectilinear, in 
accordance with the degree of brightness of the light — 
firstly, by the longer axes of the elliptical figures, which 
the plant continues to describe as long as the light re- 
mains very dim, being directed more or less accurately 
toward its source, and by each successive ellipse being 
described nearer to the light. Secondly, if the light is 
only somewhat dimmed, by the acceleration and increase 
of the movement towards it, and by the retardation or 
arrestment of that from the light, some lateral move- 
ment being still retained, for the light will interfere less 
with a movement at right angles to its direction, than 
with one in its own direction.* The result is that the 
course is rendered more or less zigzag and unequal in 



'•■ In his paper, ' Ueber ortho- heliotropism are affected by dif- 

trope und plagiotrope Pflanzen- ferences in tlie angles at which 

theile' ('Arbeiten des Bot. Inst. the organs of plants stand with 

in Wiirzburg,' Band ii. Heft ii. respect to the direction of the in- 

1879), Sachs has discussed the cident force. 
manner in which geotropism and 



Chap. VIII. AND HELIOTROPISM. 443 

rate. Lastly, when the light is very bright all lateral 
movement is lost ; and the whole energy of the plant is 
expended in rendering the circumnntating movement 
rectilinear and rapid in one direction alone, namely, 
towards the light. 

The common view seems to be that heliotropism is a 
qnite distinct kind of movement from circumnutation ; 
and it may be nrged that in the foregoing diagrams we 
see heliotropism merely combined with, or superimposed 
on, circumnutation. But if so, it must be assumed that 
a bright lateral light completely stops circumnutation, 
for a plant thus exposed moves in a straight line towards 
it, without describing any ellipses or circles. If the light 
be somewhat obscured, though amply sufficient to cause 
the plant to bend towards it, we have more or less plain 
evidence of still-continued circumnutation. It must fur- 
ther be assumed that it is only a lateral light which has 
this extraordinary power of stopping circumnutation, for 
we know that the several plants above experimented on, 
and all the others which were observed by us whilst 
growing, continue to circumnutate, however bright the 
light may be, if it comes from above. Nor should it be 
forgotten that in the life of each plant, circumnutation 
precedes heliotropism, for hypocotyls, epicotyls, and peti- 
oles circumnutate before they have broken through the 
ground and have ever felt the influence of light. 

We are therefore fully justified, as it seems to us, in 
believing that whenever light enters laterally, it is the 
movement of circumnutation which gives rise to, or is 
converted into, heliotropism and apheliotropism. On 
this view we need not assume against all analogy that 
a lateral light entirely stops circumnutation; it merely 
excites the plant to modify its movement for a time in 
a beneficial manner. The existence of every possible 
gradation, between a straight course towards a lateral 



4M MODIFIED CIRCUMNUTATION. Chap. VIII. 

light and a course consisting of a series of loops or 
ellipses, becomes perfectly intelligible. Finally, the con- 
version of circumnutation into heliotropism or apheli- 
otropism, is closely analogous to what takes place with 
sleeping plants, which during the daytime describe one 
or more ellipses, often moving in zigzag lines and mak- 
ing little loops ; for when they begin in the evening to 
go to sleep, they likewise expend all their energy in ren- 
dering their course rectilinear and rapid. In the case of 
sleep-movements, the exciting or regulating cause is a 
difference in the intensity of the light, coming from 
above, at different periods of the twenty-four hours ; 
whilst with heliotropic and apheliotropic movements, it 
is a difference in the intensity of the light on the two 
sides of the plant. 

Transversal-lieliotropismus {of Frmih^) or Diaheli- 
otropisin. — The cause of leaves placing themselves more 
or less transversely to the light, with their upper surfaces 
directed towards it, has been of late the subject of much 
controversy. We do not here refer to the object of the 
movement, which no doubt is that their upper surfaces 
may be fully illuminated, but the means by which this 
position is gained. Hardly a better or more simple in- 
stance can be given of diaheliotropism than that offered 
by many seedlings, the cotyledons of which are extended 
horizontally. When they first burst from their seed- 
coats they are in contact and stand in various positions, 
often vertically upwards ; they soon diverge, and this is 
effected by epinasty, which, as we have seen, is a modi- 
fied form of circumnutation. After they have diverged 
to their full extent, they retain nearly the same position, 



"•■•'Die naturliclie Wagerechte Frage fiber TraiisTersal-Geo-und 

Eichtuiig Yon Pflanzentheilen.' Heliotropismus," ' Bot. Zeitung,' 

1870. See also some interesting 1873, p. 17 et seq. 
articleG by the same author, " Zur 



Chap. VIII. DIAHELIOTROPISM. 445 

though brightly illuminated all day long from above, 
with their lower surfaces close to the ground and thus 
much shaded. There is therefore a great contrast in the 
degree of illumination of their upper and lower surfaces, 
and if they were heliotropic they would bend quickly 
upwards. It must not, however, be supposed that such 
cotyledons are immovably fixed in a horizontal position. 
When seedlings are exposed before a window, their hypo- 
cotyls, which are highly heliotropic, bend quickly towards 
it, and the upper surfaces of their cotyledons still remain 
exposed at right angles to the light ; but if the hypo- 
cotyl is secured so that it cannot bend, the cotyledons 
themselves change their position. If the two are placed 
in the line of the entering light, the one furthest from 
it rises up and that nearest to it often sinks down ; if 
placed transversely to the light, they twist a little later- 
ally ; so that in every case they endeavour to place their 
upper surfaces at right angles to the light. So it notori- 
ously is with the leaves on plants nailed against a wall, 
or grown in front of a window. A moderate amount of 
light suffices to induce such movements ; all that is 
necessary is that the light should steadily strike the 
plants in an oblique direction. With respect to the 
above twisting movement of cotyledons, Frank has given 
many and much more striking instances in the case of 
the leaves on branches which had been fastened in vari- 
ous positions or turned upside down. 

In our observations on the cotyledons of seedling 
plants, we often felt surprise at their persistent hori- 
zontal position during the day, and were convinced be- 
fore we had read Frank's essay, that some special ex- 
planation was necessary. De Vries has shown * that the 
more or less horizontal position of leaves is in most cases 



«- ' Arbeiteu dcs Bot. Instituts in Wuvz.,' Heft ii. 1872, pp. 223-277. 



446 MODIFIED CIRCUMNUTATION. Chap. VIII. 

influenced by epinasty, by their own weight, and by apo- 
geotropism. A young cotyledon or leaf after bursting 
free is brought down into its proper position, as already 
remarked, by epinasty, which, according to De Varies, 
long continues to act on the midribs and petioles. 
Weight can hardly be influential in the case of cotyle- 
dons, except in a few cases presently to be mentioned, 
but must be so with large and thick leaves. With re- 
spect to apogeotropism, De Vries maintains that it gen- 
erally comes into play, and of this fact we shall presently 
advance some indirect evidence. But over these and 
other constant forces we believe that there is in many 
eases, but we do not say in all, a preponderant tendency 
in leaves and cotyledons to place themselves more or less 
transversely with respect to the light 

In the cases above alluded to of seedlings exposed to 
a lateral light with their hypocotyls secured, it is im- 
possible that epinasty, weight and apogeotropism, either 
in opposition or combined, can be the cause of the rising 
of one cotyledon, and of the sinking of the other, since 
the forces in question act equally on both ; and since 
epinasty, weight and apogeotropism all act in a vertical 
plane, they cannot cause the twisting of the petioles, 
which occurs in seedlings under the above conditions of 
illumination. All these movements evidently depend in 
some manner on the obliquity of the light, but cannot 
be called heliotropic, as this implies bending towards the 
light; whereas the cotyledon nearest to the light bends 
in an opposed direction or downwards, and both place 
themselves as nearly as possible at right angles to the 
light. The movement, therefore, deserves a distinct 
name. As cotyledons and leaves are continually oscil- 
lating up and down, and yet retain all day long their 
proper position with their upper surfaces directed trans- 
versely to the light, and if displaced reassume this posi- 



Chap. YIII. DIAHELIOTROPISM. 447 

tion, dialieliotropism must be considered as a modified 
form of circumnutation. Tliis was often evident when 
the movements of cotyledons standing in front of a win- 
dow were traced. We see something analogous in the 
case of sleeping leaves or cotyledons, which after oscil- 
lating up and down during the whole day, rise into a 
vertical position late in the evening, and on the follow- 
ing morning sink down again into their horizontal or 
diaheliotropic position, in direct opposition to heliotro- 
pism. This return into their diurnal position, which 
often requires an angular movement of 90°, is analogous 
to the movement of leaves on displaced branches, which 
recover their former positions. It deserves notice that 
any force such as apogeotropism, will act with different 
degrees of power* in the different positions of those 
leaves or cotyledons which oscillate largely up and down 
during the day ; and yet they recover their horizontal or 
diaheliotropic position. 

We may therefore conclude that diaheliotropic move- 
ments cannot be fully explained by the direct action of 
light, gravitation, weight, &c., any more than can the 
nyctitropic movements of cotyledons and leaves. In the 
latter case they place themselves so that their upper sur- 
faces may radiate at night as little as possible into open 
space, with the upper surfaces of the opposite leaflets 
often in contact. These movements, which are some- 
times extremely complex, are regulated, though not di- 
rectly caused, by the alternations of light and darkness. 
In the case of diaheliotropism, cotyledons and leaves 
place themselves so that their upper surfaces may be 
exposed to the light, and this movement is regulated, 
though not directly caused, by the direction whence the 
light proceeds. In both cases the movement consists 



* See former note, in reference to Sachs' remarks on this suhjcct. 



448 MODIFIED CIRCUMNUTATION. Chap. VIII. 

of circumnutation modified by innate or constitutibnal 
causes, in the same manner as with climbing plants, the 
circumnutation of which is increased in amplitude and 
rendered more circular, or again with very young cotyle- 
dons and leaves which are thus brought down into a 
horizontal position by epinasty. 

We have hitherto referred only to those leaves and 
cotyledons which occupy a permanently horizontal posi- 
tion ; but many stand more or less obliquely, and some 
few upright. The cause of these differences of position 
is not known ; but in accordance with Wiesner's views, 
hereafter to be given, it is probable that some leaves and 
cotyledons would suffer, if they were fully illuminated 
by standing at right angles to the light. 

We have seen in the second and fourth chapters that 
those cotyledons and leaves which do not alter their 
positions at night sufficiently to be said to sleep, com- 
monly rise a little in the evening and fall again on the 
next morning, so that they stand during the night at a 
rather higher inclination than during the middle of the 
day. It is incredible that a rising movement of 2° or 3°, 
or even of 10° or 20°, can be of any service to the plant, 
so as to have been specially acquired. It must be the 
result of some periodical change in the conditions to 
which they are subjected, and there can hardly be a 
doubt, that this is the daily alternations of light and 
darkness. De Vries states in the paper before referred 
to, that most petioles and midribs are apogeotropic ; * and 



* According to Frank ('Die but when organs have been kept 

nat. Wagerechte Ricbtung von in tbe dark, the amount of water 

Pflanzentbeilen,' 1870, p. 46) the and of mineral matter which they 

root-leaves of many plants, kept contain is so much altered, and 

in darkness, rise up and even be- their regular growth is so much 

come vertical ; and so it is in some disturbed, that it is perhaps rash 

cases with shoot. (See Rauwen- to infer from their movements 

hoff. 'Archives Neerlandaises, ' what would occur under normal 

torn. xii. p. 32.) These move- conditions. fSee Godlewski, ' Bot. 

ments indicate apogeotropism ; Zeitung,' Feb, 14th, 1879.) 



Chap. VIII. DIAHELIOTROPISM. 449 

apogeotropism would account for the above rising move- 
ment, which is common to so many widely distinct spe- 
cies, if we suppose it to be conquered by diaheliotropism 
during the middle of the day, as long as it is of impor- 
tance to the plant that its cotyledons and leaves should 
be fully exposed to the light. The exact hour in the 
afternoon at which they begin to bend slightly upwards, 
and the extent of the movement, will depend on their 
degree of sensitiveness to gravitation and on their power 
of resisting its action during the middle of the day, as 
well as on the amplitude of their ordinary circumnutat- 
ing movements ; and as these qualities differ much in 
different species, we might expect that the hour in the 
afternoon at which they begin to rise would differ much 
in different species, as is the case. Some other agency, 
however, besides apogeotropism, must come into play, 
either directly or indirectly, in this upward movement. 
Thus a young bean ( Vicia faM) , growing in a small pot, 
was placed in front of a window in a klinostat ; and at 
night the leaves rose a little, althoug-h the action of apo- 
geotropism was quite eliminated. Nevertheless, they did 
not rise nearly so much at night, as when subjected to 
apogeotropism. Is it not possible, or even probable, that 
leaves and cotyledons, which have moved upwards in the 
evening through the action of apogeotropism during 
countless generations, may inherit a tendency to this 
movement? We have seen that the hypocotyls of sev- 
eral Leguminous plants have from a remote period in- 
herited a tendency to arch themselves; and we know 
that the sleep-movements of leaves are to a certain ex- 
tent inherited, independently of the alternations of light 
and darkness. 

In our observations on the circumnutation of those 
cotyledons and leaves which do not sleep at night, we 
met with hardly any distinct cases of their sinking a 



450 MODIFIED CIRCUMNUTATION. Chap. VIII. 

little in the evening, and rising again in the morning, — 
that is, of movements the reverse of those just discussed. 
We have no doubt that such cases occur, inasmucii as 
the leaves of many plants sleep by sinking vertically 
downwards. How to account for the few cases which 
were observed must be left doubtful. The young leaves 
of Cannabis sativa sink at night between 30° and 40° 
beneath the horizon ; and Kraus attributes this to epi- 
nasty in conjunction with the absorption of water. 
Whenever epinastic growth is vigorous, it might con- 
quer diaheliotropism in the evening, at which time it 
would be of no importance to the plant to keep its leaves 
horizontal. The cotyledons of Anoda WrigJitii, of one 
variety of Gossypium, and of several species of Ipomoea, 
remain horizontal in the evening whilst they are very 
young; as they grow a little older they curve a little 
downwards, and when large and heavy sink so much that 
they come under our definition of sleep. In the case of 
the Anoda and of some species of Ipomoea, it was proved 
that the downward movement did not depend on the 
weight of the cotyledons ; but from the fact of the move- 
ment being so much more strongly pronounced after the 
cotyledons have grown large and heavy, we may suspect 
that their weight aboriginally played some part in de- 
termining that the modification of the circumnutating 
movement should be in a downward direction. 

The so-called Diurnal Sleep of Leaves, or Paraheli- 
otropism. — This is another class of movements, dependent 
on the action of light, which supports to some extent 
the belief that the movements above described are only 
indirectly due to its action. We refer to the movements 
of leaves and cotyledons which when moderately illumi- 
nated are diaheliotropic ; but which change their posi- 
tions and present their edges to the light, when the sun 
shines brightly on them. These movements have some- 



Chap. VIII. PARAHELIOTROPISM. 451 

times been called diurnal sleep, but they differ wholly 
with respect to the object gained from those properly 
called nyctitropic ; and in some cases the position occu- 
pied during the day is the reverse of that during the 
night. 

It has long been known * that when the sun shines brightly 
on the leaflets of Robinia, they rise up and present their edges to 
the light; whilst their position at night is vertically downwards. 
We have observed the same movement, when the sun shone 
brightly on the leaflets of an Australian Acacia. Those of 
AmphicarpcBa monoica turned their edges to the sun; and an 
analogous movement of the httle almost rudimentary basal leaf- 
lets of Mimosa alhida was on one occasion so rapid that it could 
be distinctly seen through a lens. The elongated, unifoliate, 
first leaves of Phaseolus Boxburghii stood at 7 a.m. at 20° above 
the horizon, and no doubt they afterwards sank a little lower. 
At noon, after having been exposed for about 2 h. to a bright 
sun, they stood at 56° above the horizon; they were then pro- 
tected from the rays of the sun, but were left well illuminated 
from above, and after 30 m. they had fallen 40°, for they now 
stood at only 16° above the horizon. Some young plants of 
Phaseolus Hernandesii had' been exposed to the same bright sun- 
light, and their broad, unifoliate, first leaves now stood up 
almost or quite vertically, as did many of the leaflets on the 
trifoliate secondary leaves; but some of the leaflets had twisted 
round on their own axes by as much as 90° without rising, so as 
to present their edges to the sun. The leaflets on the same 
leaf sometimes behaved in these two different manners, but 
always with the result of being less intensely illuminated. 
These plants were then protected from the sun, and were looked 
at after 1^ h. ; and now all the leaves and leaflets had reassumed 
their ordinary sub-horizontal positions. The copper-coloured 
cotyledons of some seedlings of Cassia mimosoides were horizon- 
tal in the morning, but after the sun had shone on them, each 
had risen 45J^° above the horizon. The movement in these 



* Pfeffer prives the names and his 'Dip Poriodischen Bcwcgun- 
dates of several ancient writers in gen,' 1875, p. 62. 



452 



MODIFIED CIRCUMNUTATION. Chap. VIII. 



several cases must not be confounded with the sudden closing 
of the leaflets of Mimosa pudica, which may sometimes be noticed 
when a plant which has been kept in an obscure place is sud- 
denly exposed to the sun ; for in this case the light seems to act, 
as if it were a touch. 

From Prof. Wiesner's interesting observations, it is probable 
that the above movements have been acquired for a special 
purpose. The chlorophyll in leaves is often injured by too 
intense a light, and Prof. Wiesner* believes that it is protected 
by the most diversified means, such as the presence of hairs, 
colouring matter, &c., and amongst other means by the leaves 
presenting their edges to the sun, so that the blades then receive 
much less light. He experimented on the young leaflets of 
Robinia, by fixing them in such a position that they could not 
escape being intensely illuminated, whilst others were allowed 
to place themselves obliquely; and the former began to suffer 
from the light in the course of two days. 

Fig. 180. 




Averrhoa lilimhi: leaf with leaflets depressed after exposure to sun- 
shine ; but the leaflets are sometiines more depressed than is here 
shown. Figure much reduced. 



In the cases above given, the leaflets move either upwards 
or twist laterally, so as to place their edges in the direction of the 
sun's light; but Cohn long ago observed that the leaflets of 
Oxalis bend downwards when fully exposed to the sun. We 
witnessed a striking instance of this movement in the very large 



*" ' Die Naturlichen Einrich- 
tungen zum Schutze des Chloro- 
phylls.' &c., 1876. Prine:sheim 
has recently observed under the 
microscope the destruction of 
chlorophyll in a few minutes by 



the action of concentrated light 
from the sun, in the presence of 
oxygen. See, also, Stahl on the 
protection of chlorophyll from 
intense light, in ' Bot. Zeitung, ' 
1880. 



Chap. VIII. PARAHELIOTROPISM. 453 

leaflets of 0. Ortegesii. A similar movement may frequently be 
observed with the leaflets of Aoerrhoa hilimhi (a member of tlie 
Oxaliclse) ; and a leaf is here represented (Fig. 180) on which 
the sun had shone. A diagram (Fig. 134) was given in the last 
chapter, representing the oscillations by which a leaflet rapidly 
descended under these circumstances; and the movement may 
be seen closely to resemble that (Fig. 183) by which it assumed 
its nocturnal position. It is an interesting fact in relation to 
our present subject that, as Prof. Batalin informs us in a letter 
dated February, 1879, the leaflets of Oxalis acetoseUa may be 
daily exposed to the sun during many weeks, and they do not 
suffer if they are allowed to depress themselves ; but if this be pre- 
vented, they lose their colour and wither in two or three days. 
Yet the duration of a leaf is about two months, when subjected 
only to diffused light ; and in this case the leaflets never sink 
downwards during the day. 

As the upward movements of the leaflets of Robinia, 
and the downward movements of those of Oxalis, have 
been proved to be highly beneficial to these plants when 
subjected to bright sunshine, it seems probable that they 
have been acquired for the special purpose of avoiding 
too intense an illumination. As it would have been very 
troublesome in all the above cases to have watched for a 
fitting opportunity and to have traced the movement of 
the leaves whilst they were fully exposed to the sunshine, 
we did not ascertain whether paraheliotropism always 
consisted of modified circumnutation ; but this certainly 
was the case with the Averrhoa, and probably with the 
other species, as their leaves were continually circum- 
nutating. 



CHAPTER IX. 

Sensitiveness of Plants to Light: its transmitted 

EFFECTS. 

Uses of heliotropism — Insectivorous and climbing plants not helio- 
tropic — Same organ heliotropic at one age and not at another — 
Extraordinary sensitiveness of some plants to light — The effects 
of light do not correspond with its intensity — Effects of previous 
illumination — Time required for the action of light — After-effects 
of light — Apogeotropism acts as soon as light fails — Accuracy with 
which plants bend to the light — This dependent on the illumina- 
tion of one whole side of the part — Localised sensitiveness to light 
and its transmitted effects — Cotyledons of Phalaris, manner of 
bending — Results of the exclusion of light from their tips — Effects 
transmitted beneath the surface of the ground — Lateral illumina- 
tion of the tip determines the direction of the curvature of the 
base — Cotyledons of Avena, curvature of basal part due to the 
illumination of upper part — Similar results with the hypocotyls of 
Brassica and Beta — Eadicles of Sinapis apheliotropic, due to the 
sensitiveness of their tips— Concluding remarks and summary of 
chapter — Means by which circumnutation has been converted into 
heliotropism or apheliotropism. 

'No one can look at the plants growing on a bank or 
on the borders of a thick wood, and doubt that the 
young stems and leaves place themselves so that the 
leaves may be well illuminated. They are thus enabled 
to decompose carbonic acid. But the sheath-like coty- 
ledons of some Gramine^, for instance, those of Phalaris, 
are not green and contain very little starch ; from which 
fact we may infer that they decompose little or no car- 
bonic acid. Nevertheless, they are extremely heliotropic ; 
and this probably serves them in another way, namely, as 
a guide from the buried seeds through fissures in the 
ground or through overlying masses of vegetation, into 
454 



Chap. IX. SENSITIVENESS TO LIGHT. 455 

the light and air. This view is strengthened by the fact 
that with Phalaris and Avena the first true leaf, which is 
bright green and no doubt decomposes carbonic acid, 
exhibits hardly a trace of heliotropism. The heliotropic 
movements of many other seedlings probably aid them 
in like manner in emerging from the ground ; for apoge- 
otropism by itself would blindly guide them upwards, 
against any overlying obstacle. 

Heliotropism prevails so extensively among the higher 
plants, that there are extremely few, of which some part, 
either the stem, flower-peduncle, petiole, or leaf, does not 
bend towards a lateral light. Drosera rotiindifolia is one 
of the few plants the leaves of which exhibit no trace of 
heliotropism. Nor could we see any in Dionaea, though 
the plants were not so carefully observed. Sir J. Hooker 
exposed the pitchers of Sarracenia for some time to a 
lateral light, but they did not bend towards it.* We can 
understand the reason why these insectivorous plants 
should not be heliotropic, as they do not live chiefly by 
decomposing carbonic acid ; and it is much more impor- 
tant to them that their leaves should occupy the best 
position for capturing insects, than that they should be 
fully exposed to the light. 

Tendrils, which consist of leaves or of other oi'gans 
modified, and the stems of twining plants, are, as Mohl 
long ago remarked, rarely heliotropic ; and here again we 
can see the reason why, for if they had moved towards 
a lateral light they would have been drawn away from 
their supports. But some tendrils are apheliotropic, for 
instance those of Bignonia caiweolata and of Sinilax 
aspera; and the stems of some plants which climb by 



*■ According to F. Kurtz (" Ver- tonia CaHfornica are strongly 

handl. des Bot. Vereins der Pro- apheliotropic. We failed to detect 

vinz Brandenburg,' Bd. xx. 1878) this movement in a plant which 

the leaves or pitchers of Darling- we possessed for a short time. 

80 



456 SENSITIVENESS TO LIGHT. Chap. IX. 

rootlets, as tliose of the Ivy and Tecoma radicans^ are 
likewise apheliotropic, and they thus find a support. 
The leaves, on the other hand, of most climbing plants 
are heliotropic ; but we could detect no signs of any such 
movement in those of Mutisia clematis.. 

As heliotropism is so widely prevalent, and as twin- 
ing plants are distributed throughout the whole vascular 
series, the apparent absence of any tendency in their stems 
to bend towards the ligiit, seemed to us so remarkable a 
fact as to deserve further investigation, for it implies 
that heliotropism can be readily eliminated. When twin- 
ing plants are exposed to a lateral light, their stems go 
on revolving or circumnutaiing about the same spot, 
without any evident deflection towards the light ; but 
we thought that we might detect some trace of heli- 
otropism by comparing the average rate at which the 
stems moved to and from the light during their succes- 
sive revolutions.* Three young plants (about a foot in 
height) of Ipomcea ccBrulea and four of /. purpurea.^ 
growing in separate pots, were placed on a bright day 
before a north-east window in a room otherwise dark- 
ened, with the tips of their revolving stems fronting the 
window. When the tip of each plant pointed directly 
from the window, and when again towards it, the times 
were recorded. This was continued from 6.45 a.m. till a 
little after 2 p.m. on June 17th. After a few observa- 
tions we concluded that we could safely estimate the 
time taken by each semicircle, within a limit of error of 
at most 5 minutes. Although the rate of movement in 
different parts of the same revolution varied greatly, yet 



*■ Some erroneous statements number of observations, for we did 

are unfortunately given on this not then know at how unequal 

subject, in 'The Movements and a rate the stems and tendrils of 

Habits of Climbing Plants,' 1875, climbing plants sometimes travel 

pp. 28, 32, 40, and 53. Conclusions in diiferent parts of the same revo- 

were drawn from an insufficient lution. 



Chap. IX. SENSITIVENESS TO LIGHT. 457 

22 semicircles to the light were completed, each on an 
average in 7395 minutes; and 22 semicircles from the 
light each in 73-5 minntes. It may, therefore, be said 
that they travelled to and from the light at exactly the 
same average rate ; though probably the accuracy of the 
result was in part accidental. In the evening the stems 
were not in the least deflected towards the window. 
Nevertheless, there appears to exist a vestige of heli- 
otropism, for with 6 out of the 7 plants, the first semicir- 
cle from the light, described in the early morning after 
they had been subjected to darkness during the night 
and thus probably rendered more sensitive, required 
rather more time, and the first semicircle to the light 
considerably less time, than the average. Thus with all 
7 plants, taken together, the mean time of the first semi- 
circle in the morning from tlie light, was 76-8 minutes, 
instead of 73-5 minutes, which is the mean of all the 
semicircles during the day from the light ; and the mean 
of the first semicircle to the light was only 63-1, instead 
of 73-95 minutes, which was the mean of all the semicir- 
cles during the day to the light. 

Similar observations were made on Wistaria Sinensis^ 
and the mean of 9 semicircles from the light was 117 
minutes, and of 7 semicircles to the light 122 minutes, 
and this difference does not exceed the probable limit of 
error. During the three days of exposure, the shoot did 
not become at all bent towards the window before which 
it stood. In this case the first semicircle from the light 
in the early morning of each day, required rather less 
time for its performance than did the first semicircle to 
the light ; and this result, if not accidental, appears to 
indicate that the shoots retain a trace of an original 
apheliotropic tendency. With Lonicera hrachypoda the 
semicircles from and to the light differed considerably in 
time ; for 5 semicircles from the light required on a mean 



458 SENSITIVENESS TO LIGHT. Chap. IX. 

202*4 minutes, and 4 to the light, 229'5 minutes ; but the 
shoot moved very irregularly, and under these circum- 
stances the observations were much too few. 

It is remarkable that the same part on the same plant 
may be affected by light in a widely different manner 
at different ages, and as it appears at different seasons. 
The hypocotyledonous stems of Ipomma cmrulea and 
puvpuvea are extremely helio tropic, whilst the stems of 
older plants, only about a foot in height, are, as we have 
just seen, almost wholly insensible to light. Sachs states 
(and we have observed the same fact) that the hypocotyls 
of the Ivy {Hedera helix) are slightly heliotropic ; where- 
as the stems of plants grown to a few inches in height 
become so strongly apheliotropic, that they bend at right 
angles away from the light. Nevertheless, some young 
plants Avhich had behaved in this manner early in the 
summer again became distinctly heliotropic in the be- 
ginning of September ; and the zigzag courses of their 
stems, as they slowly curved towards a north-east Avin- 
dow, were traced during 10 days. The stems of very 
young plants of Tropmolum majus are highly heliotropic, 
whilst those of older plants, according to Sachs, are 
slightly apheliotropic. In all these cases the heliotropism 
of the very young stems serves to expose the cotyledons, 
or when the cotyledons are hypogean the first true leaves, 
fully to the light ; and the loss of this power by the older 
stems, or their becoming apheliotropic, is connected with 
their habit of climbing. 

Most seedling plants are strongly heliotropic, and it 
is no doubt a great advantage to them in their struggle 
for life to expose their cotyledons to the light as quickly 
and as fully as possible, for the sake of obtaining carbon. 
It has been shown in the first chapter that the greater 
number of seedlings circumnutate largely and rapidly ; 
and as heliotropism consists of modified circumnuta- 



Chap. IX. SENSITIVENESS TO LIGHT. 459 

tion, we are tempted to look at the high development of 
these two powers in seedlings as intimately connected. 
Whether there are any plants which circumnutate slowly 
and to a small extent, and yet are highly heliotropic, we 
do not know; but there are several, and there is nothing 
surprising in this fact, which circumnutate largely and 
are not at all, or only slightly, heliotropic. Of such cases 
Drosera rotiuulifulia oifers an excellent instance. The 
stolons of the strawberry circumnutate almost like the 
stems of climbing plants, and they are not at all affected 
by a moderate light ; but when exposed late in the sum- 
mer to a somewhat brighter light they were slightly 
heliotropic ; in sunlight^ according to De Vries, they are 
apheliotropic. Climbing plants circumnutate much 
more widely than any other plants, yet they are not at 
all heliotropic. 

Although the stems of most seedling plants are 
strongly heliotropic, some few are but slightly helio- 
tropic, without our being able to assign any reason. 
This is the case with the hypocotyl of Cassia tora, and 
we were struck with the same fact with some other seed- 
lings, for instance, those of Reseda odorata. With re- 
spect to the degree of sensitiveness of the more sensitive 
kinds, it was shown in the last chapter that seedlings of 
several species, placed before a north-east window pro- 
tected by several blinds, and exposed in the rear to the 
diffused light of the room, moved with unerring cer- 
tainty towards the window, although it was impossible 
to judge, excepting by the shadow cast by an upright 
pencil on a white card, on which side most light en- 
tered, so that the excess on one side must have been 
extremely small. 

A pot with seedlings of Plialaris Caviar iensis^ which 
had been raised in darkness, was placed in a completely 
darkened room, at 12 feet from a very snuill lamp. 



460 SENSITIVENESS TO LIGHT. Chap. IX. 

After 3 h. the cotyledons were doubtfully curved to- 
wards the light, and after 7 h. 40 m. from the first 
exposure, they were all plainly, though slightly, curved 
towards the lamp. Now, at this distance of 12 feet, the 
light was so obscure that we could not see the seedlings 
themselves, nor read the large Koman figures on the 
white face of a watch, nor see a pencil line on paper, but 
could just distinguish a line made with Indian ink. It 
is a more surprising fact that no visible shadow was cast 
by a pencil held upright on a white card ; the seedlings, 
therefore, were acted on by a difl:erence in the illumina- 
tion of their two sides, which the human eye could not 
distinguish. On another occasion even a less degree of 
light acted, for some cotyledons of Phalaris became 
slightly curved towards the same lamp at a distance of 
20 feet ; at this distance we could not see a circular dot 
2-29 mm. (-09 inch) in diameter made with Indian ink 
on white paper, though we could just see a dot 3-56 mm. 
(14 inch) in diameter; yet a dot of the former size 
appears large when seen in the light.* 

We next tried how small a beam of light would act ; 
as this bears on light serving as a guide to seedlings 
whilst they emerge through fissured or encumbered 
ground. A pot with seedlings of Phalaris was covered 
by a tin-vessel, having on one side a circular hole 1*23 
mm. in diameter (i.e. a little less than the -gioth of an 
inch) ; and the box was placed in front of a paraffin 
lamp and on another occasion in front of a window ; and 
both times the seedlings were manifestly bent after a few 
hours towards the little hole. 

A more severe trial was now made; little tubes of 
very thin glass, closed at their upper ends and coated 



* Strasburger says ('Wirkung Hpematococeus moved to a light 
des Lichtes auf Schwarmsporen,' which only just sufficed to allow 
1878, p. 52), that the spores of middle-sized type to be read. 



Chap. IX. SENSITIVENESS TO LIGHT. 461 

with black varnish, were slipped over the cotyledons of 
Phaiaris (which had germinated in darkness) and just 
fitted them. Narrow stripes of the varnish had been 
previously scraped off one side, through which alone 
light could enter ; and their dimensions were afterwards 
measured under the microscope. As a control experi- 
ment, similar unvarnished and transparent tubes were 
tried, and they did not prevent the cot3'ledons bending 
towards the light. Two cotyledons were placed before a 
south-west window, one of which was illuminated by a 
stripe in the varnish, only -004 inch (0-1 mm.) in breadth 
and -016 inch (0-4 mm.) in length ; and the other by a 
stripe -008 inch in breadth and -06 inch in length. The 
seedlings were examined after an exposure of 7 h. 40 m., 
and were found to be manifestly bowed towards the 
light. Some other cotyledons were at the same time 
treated similarly, excepting that the little stripes were 
directed not to the sky, but in such a manner that they 
received only the diffused light from the room ; and 
these cotyledons did not become at all bowed. Seven 
other cotyledons were illuminated through narrow, but 
comparatively long, cleared stripes in the varnish — 
namely, in breadth between -01 and -026 inch, and in 
length between -15 and -3 inch; and these all became 
bowed to the side, by which light entered through the 
stripes, whether these were directed towards the sky or 
to one side of the room. That light passing through a 
hole only -004 inch in breadth by -016 in length, should 
induce curvature, seems to us a surprising fact. 

Before we knew how extremely sensitive the cotyle- 
dons of Phaiaris were to light, we endeavoured to trace 
their circumnutation in darkness by the aid of a small 
wax taper, held for a minute or two at each observation 
in nearly the same position, a little on the left side in 
front of the vertical glass on which the tracing was made. 



462 SENSITIVENESS TO LIGHT. Chap. IX. 

Tlie seedlings were thus observed seventeen times in the 
course of the day, at intervals of from half to three- 
quarters of an hour ; a]]d late in the evening we were 
surprised to find that all the 29 cotyledons were greatly 
curved and pointed towards the vertical glass, a little to 
the left where the taper had been held. The tracings 
showed that they had travelled in zigzag lines. Thus, 
an exposure to a feeble light for a very short time at the 
above specified intervals, sufficed to induce well-marked 
heliotropism. An analogous case was observed with the 
hypocotyls of Solaniim ly coper siciim. We at first at- 
tributed this result to the af ter-eifects of the light on 
each occasion; but since reading Wiesner's observations,* 
which will be referred to in the last chapter, we cannot 
doubt that an intermittent light is more efficacious than 
a continuous one, as plants are especially sensitive to any 
contrast in its amount. 

The cotyledons of Phalaris bend much more slowly 
towards a very obscure light than towards a bright one. 
Thus, in the experiments with seedlings placed in a dark 
room at 12 feet from a very small lamp, they were just 
perceptibly and doubtfully curved towards it after 3 h., 
and only slightly, yet certainh^, after 4 h. After 8 h. 
40 m. the chords of their arcs were deflected from the 
perpendicular by an average angle of only 16°. Had the 
light been bright, they would have become much more 
curved in between 1 and 2 h. Several trials were made 
with seedlings placed at various distances from a small 
lamp in a dark room; but we will give only one trial. 
Six pots were placed at distances of 2, 4, 8, 12, 16, and 
20 feet from the lamp, before which they were left for 
4 h. As light decreases in a geometrical ratio, the seed- 
lings in the 2nd pot received Jth, those in the 3rd pot 



'Sitz. der k. Akad. der Wissensch.' (Vienna), Jan. 18S0, p. 12. 



Chap. IX. SENSITIVENESS TO LIGHT. 463 

yig-th, those in the 4th -g^th, those in the 5th -614th, and 
those in the 6th y-J-Q-th of the light received by the seed- 
lings in the first or nearest pot. Therefore it might 
have been expected that there would have been an im- 
mense difference in the degree of their heliotropic curva- 
ture in the several pots ; and there was a well-marked 
difference between those which stood nearest and fur- 
thest from the lamp, but the difference in each succes- 
sive pair of pots was extremel}^ small. In order to avoid 
prejudice, we asked three persons, who knew nothing 
about the experiment, to arrange the jDots in order ac- 
cording to the degree of curvature of the cotyledons. 
The first person arranged them in proper order, but 
doubted long between the 12 feet and 16 feet pots; yet 
these two received light in the proportion of 36 to 64. 
The second person also arranged them properly, but 
doubted between the 8 feet and 12 feet pots, which re- 
ceived light in the proportion of 16 to 36. The third 
person arranged them in wrong order, and doubted about 
four of the pots. This evidence shows conclusively how 
little the curvature of the seedlings differed in the suc- 
cessive pots, in comparison with the great difference in 
the amount of light which they received ; and it should 
be noted that there was no excess of superfluous light, 
for the cotyledons became but little and slowly curved 
even in the nearest pot. Close to the 6th pot, at the 
distance of 20 feet from the lamp, the light allowed us 
just to distinguish a dot 3-56 mm. (-14 inch) in diameter, 
made with Indian ink on white paper, but not a dot 2-;^ 9 
mm. (;09 inch) in diameter. 

The degree of curvature of the cotyledons of Phalaris 
within a given time, depends not merely on the jimount 
of lateral light which they may then receive, but on that 
which they have previously received from above and on 
all sides. Analogous facts have been given with respect 



464 SENSITIVENESS TO LIGHT. Chap. IX. 

to the nyctitropic and periodic movements of plants. Of 
two pots containing seedlings of Phalaris which had ger- 
minated in darkness, one was still kept in the dark, and 
the other was exposed (Sept. 26th) to the light in a green- 
house during a cloudy day and on the following bright 
morning. On this morning (27th), at 10.30 a.m., both 
pots were placed in a box, blackened within and open in 
front, before a north-east window, protected by a linen 
and muslin blind and by a towel, so that but little light 
was admitted, though the sky was bright. Whenever 
the pots were looked at, this was done as quickly as 
possible, and the cotyledons were then held transversely 
with respect to the light, so that their curvature could 
not have been thus increased or diminished. After 50 ra. 
the seedlings which had previously been kept in darkness, 
were perhaps, and after 70 m. were certainly, curved, 
though very slightly, towards the window. After 85 m. 
some of the seedlings, which had previously been illumi- 
nated, were perhaps little affected, and after 100 m. some 
of the younger ones were certainly a little curved towards 
the light. At this time (i.e. after 100 m.) there was a 
plain difference in the curvature of the seedlings in the 
two pots. After 2 h. 12 m. the chords of the arcs of 
four of the most strongly curved seedlings in each pot 
were measured, and the mean angle from the perpen- 
dicular of those which had previously been kept in dark- 
ness was 19°, and of those which had previously been 
illuminated only 7°. Nor did this difference diminish 
during two additional hours. As a check, the seedlings 
in both pots were then placed in complete darkness for 
two hours, in order that apogeotropism should act on 
them ; and those in the one pot which were little curved 
became in this time almost completely upright, whilst 
the more curved ones in the other pot still remained 
plainly curved. 



Chap. IX. SENSITIVENESS TO LIGHT. 4^5 

Two days afterwards the experiment was repeated, 
with the sole difference that even less light was admitted 
through the window, as it was protected by a linen and 
mnslin blind and by two towels ; the sky, moreover, was 
somewhat less bright. The result was the same as 
before, excepting that everything occurred rather slower. 
The seedlings which had been previously kept in dark- 
ness were not in the least curved after 54 m., but were 
so after 70 m. Those which had previously been illumi- 
nated were not at all affected until 130 m. had elapsed, 
and then only slightly. After 145 m. some of the seed- 
lings in this latter pot were certainly curved towards the 
light; and there was now a plain difference between the 
two pots. After 3 h. 45 m. the chords of the arcs of 3 
seedlings in each pot were measured, and the mean angle 
from the perpendicular was 16° for those in the pot which 
had previously been kept in darkness, and only 5° for 
those which had previously been illuminated. 

The curvature of the cotyledons of Phalaris towards 
a lateral light is therefore certainly influenced by the 
degree to which they have been previously illuminated. 
We shall presently see that the influence of light on their 
bending continues for a short time after the light has 
been extinguished. These facts, as well as that of the 
curvature not increasing or decreasing in nearly the same 
ratio with that of the amount of light which they receive, 
as shown in the trials with the plants before the lamp, 
all indicate that light acts on them as a stimulus, in 
somewhat the same manner as on the nervous system of 
animals, and not in a direct manner on the cells or cell- 
walls which by their contraction or expansion cause the 
curvature. 

It has already been incidentally shown how slowly the 
cotyledons of Phalaris bend towards a very dim light; 
but when they were placed before a bright paraffin lamp 



4:ee SENSITIVENESS TO LIGHT. Chap. IX. 

their tips were all curved rectangularly towards it in 2 h. 
20 m. The hypocotyls of Sulanum lycopersicum had 
bent in the morning at right angles towards a north-east 
window. At 1 p.m. (Oct. 21st) the pot was turned 
round, so that the seedlings now pointed from the light, 
but by 5 P.M. they had reversed their curvature and 
again pointed to the light. They had thus passed 
through 180° in 4 h., having in the morning previously 
passed through about 90°. But the reversal of the first 
half of the curvature will have been aided by apogeotro- 
pism. Similar cases were observed with other seedlings, 
for instance, with those of Sinapis alba. 

We attempted to ascertain in how short a time light 
acted on the cotyledons of Phalaris, but this was difficult 
on account of their rapid circumnutating movement ; 
moreover, they differ much in sensibility, according to 
age ; nevertheless, some of our observations are worth 
giving. Pots with seedlings were placed under a micro- 
scope provided with an eye-piece micrometer, of which 
each division equalled -g^th of an inch (0-051 mm.) ; 
and they were at first illuminated by light from a par- 
affin lamp passing through a solution of bichromate of 
potassium, which does not induce heliotropism. Thus 
the direction in which the cotyledons were circumnutat- 
ing could be observed independently of any action from 
the light ; and they could be made, by turning round 
the pots, to circumnutate tranversely to the line in 
which the light would strike them, as soon as the solution 
was removed. The fact that the direction of the circum- 
nutating movement might change at any moment, and 
thus the plant might bend either towards or from the 
lamp independently of the action of the light, gave an 
element of uncertainty to the results. After the solution 
had been removed, five seedlings which were circumnu- 
tating transversely to the line of light, began to move 



Chap. IX. SENSITIVENESS TO LIGHT. 467 

towards it, in 6, 4, 7|, 6, and 9 minutes. In one of these 
cases, the apex of the cotyledon crossed five of the divi- 
sions of the micrometer (i.e. 3^o"^h of an inch, or 0'254 
mm.) towards the light in 3 m. Of two seedlings which 
were moving directly from the light at the time when 
the solution was removed, one began to move towards it 
in 13 m., and the other in 15 m. This latter seedling 
was observed for more than an hour and continued to 
move towards the light ; it crossed at one time 5 divi- 
sions of the micrometer (0-254 mm.) in 2 m. 30 s. In all 
these cases, the movement towards the light was ex- 
tremely unequal in rate, and the cotyledons often re- 
mained almost stationary for some minutes, and two of 
them retrograded a little. Another seedling which was 
circumnutating transversely to the line of light, moA'ed 
towards it in 4 m. after the solution was removed ; it then 
remained almost stationary for 10 m. ; then crossed 5 
divisions of the micrometer in 6 m. ; and then 8 divisions 
in 11 m. This unequal rate of movement, interrupted 
by pauses, and at first with occasional retrogressions, 
accords well with our conclusion that heliotropism con- 
sists of modified circumnutation. 

In order to observe how long the after-effects of light 
lasted, a pot with seedlings of Phalaris, which had ger- 
minated in darkness, was placed at 10.40 a.m. before a 
north-east window, being protected on all other sides 
from the light ; and the movement of a cotyledon was 
traced on a horizontal glass. It circumnutated about 
the same space for the first 24 m., and during the next 
1 h. 33 m. moved rapidly towards the light. The light 
was now (i.e. after 1 h. 57 m.) completely excluded, but 
the cotyledon continued bending in the same direction 
as before, certainly for more than 15 m., probably for 
about 27 m. The doubt arose from the necessity of not 
looking at the seedlings often, and thus exposing them, 



468 SENSITIVENESS TO LIGHT. Chap. IX. 

though momentarily, to the light. This same seedling 
was now kept in the dark, until 2.18 p.m., by which time 
it had reacquired through apogeotropism its original 
upright position, when it was again exposed to the light 
from a clouded sky. By 3 p.m. it had moved a very 
short distance towards the light, but during the next 
45 m. travelled quickly towards it. After this exposure 
of 1 h. 27 m. to a rather dull sky, the light was again 
completely excluded, but the cotjdedon continued to 
bend in the same direction as before for 14 m. within a 
very small limit of error. It was then placed in the 
dark, and it now moved backwards, so that after 1 h. 
7 m. it stood close to where it had started from at 2.18 
P.M. These observations show that the cotyledons of 
Phalaris, after being exposed to a lateral light, continue 
to bend in the same direction for between a quarter and 
half an hour. 

In the two experiments just given, the cotyledons 
moved backwards or from the window shortly after 
being subjected to darkness ; and whilst tracing the cir- 
cumnutation of various kinds of seedlings exposed to a 
lateral light, we repeatedly observed that late in the 
evening, as the light waned, they moved from it. This 
fact is shown in some of the diagrams given in the last 
chapter. We wished therefore to learn whether this was 
wholly due to apogeotropism, or whether an organ after 
bending towards the light tended from any other cause 
to bend from it, as soon as the light failed. According- 
ly, two pots of seedling Phalaris and one pot of seedling 
Brassica were exposed for 8 h. before a paraffin lamp, by 
which time the cotyledons of the former and the hypo- 
cotyls of the latter were bent rectangularly towards the 
light. The pots were now quickly laid horizontally, so 
that the upper parts of the cot3dedons and of the hypo- 
cotyls of 9 seedlings projected vertically upwards, as 



Chap. IX. SENSITIVENESS TO LIGHT. 469 

proved by a plumb line. In this position they could not 
be acted on by apogeotropism, and if they possessed any 
tendency to straighten themselves or to bend in opposi- 
tion to their former heliotropic curvature, this would be 
exhibited, for it would be opposed at first very slightly 
by apogeotropism. They w^ere kept in the dark for 4 h., 
during which time they were twice looked at; but no 
uniform bending in opposition to their former helio- 
tropic curvature could be detected. We have said uni- 
form bending, because they circumnutated in their new 
position, and after 2 h. were inclined in different direc- 
tions (between 4° and 11°) from the perpendicular. 
Their directions were also changed after two additional 
hours, and again on the following morning. We may 
therefore conclude that the bending back of plants from 
a light, -when this becomes obscure or is extinguished, is 
wholly due to apogeotropism.* 

In our various experiments we were often struck with 
the accuracy with which seedlings pointed to a light 
although of small size.' To test this, many seedlings of 
Phalaris, which had germinated in darkness in a very 
narrow box several feet in length, were placed in a dark- 
ened room, near to and in front of a lamp having a small 
cylindrical wick. The cotyledons at the two ends and 
in the central part of the box, would therefore have to 
bend in widely different directions in order to point to 
the light. After they had become rectangularly bent, a 
long white thread was stretched by two persons, close 
over and parallel, first to one and then to another cotyle- 
don ; and the thread was found in almost every case 
actually to intersect the small circular wick of the now 



* It appears from a reference heliotropically is at the same time 

in Wiesner ('Die Undulirende striving, through apogeotropism, 

Nutation der Intcruodicn,' p. 7), to raise itself into a vertical posi- 

that H. Miiller of Thurgau found tion. 
that a stem which is bending 



470 SENSITIVENESS TO LIGHT. Chap. IX. 

extinguislied lamp. The deviation from accuracy never 
exceeded, as far as we could judge, a degree or two. 
This extreme accuracy seems at first surprising, but is 
not really so, for an upright cylindrical stem, whatever 
its position may be with respect to the light, would have 
exactly half its circumference illuminated and half in 
shadow ; and as the difference in illumination of the two 
sides is the exciting cause of heliotropism, a cylinder 
would naturally bend with much accuracy towards the 
light. The cotyledons, however, of Phalaris are not 
cylindrical, but oval in section ; and the longer axis was 
to the shorter axis (in the one which was measured) as 
100 to 70. Nevertheless, no difference could be detected 
in the accuracy of their bending, whether they stood 
with their broad or narrow sides facing the light, or in 
any intermediate position ; and so it was with the cotyle- 
dons of Avena sativa^ which are likewise oval in section. 
Now, a little reflection will show that in whatever posi- 
tion the cotyledons may stand, there will be a line of 
greatest illumination, exactly fronting the light, and on 
each side of this line an equal amount of light will be 
received; but if the oval stands obliquely with respect 
to the light, this will be diffused over a wider surface on 
one side of the central line than on the other. We may 
therefore infer that the same amount of light, whether 
diffused over a wider surface or concentrated on a smaller 
surface, produces exactly the same effect ; for the cotyle- 
dons in the long narrow box stood in all sorts of posi- 
tions with reference to the light, yet all pointed truly 
towards it. 

That the bending of the cotyledons to the light de- 
pends on the illumination of one whole side or on the 
obscuration of the whole opposite side, and not on a 
narrow longitudinal zone in. the line of the light being 
affected, was shown by the effects of painting longitudi- 



Chap. IX. SENSITIVENESS TO LIGHT. 471 

nally with Indian ink one side of five cotyledons of 
Phalaris. These were then placed on a table near to a 
south-west window, and the painted half was directed 
either to the right or left. The result was that instead 
of bending in a direct line towards the window, they 
were deflected from the window and towards the un- 
painted side, by the following angles, 35°, 83°, 31°, 43°, 
and 39°. It should be remarked that it was hardly pos- 
sible to paint one-half accurately, or to place all the 
seedlings which are oval in section in quite the same 
position relatively to the light ; and this will account for 
the differences in the angles. Five cotyledons of Avena 
were also painted in the same manner, but with greater 
care ; and they were laterally deflected from the line of 
the window, towards the unpainted side, by the following 
angles, 44°, 44°, 55°, 51°, and 57°. This deflection of 
the cotyledons from the window is intelligible, for the 
whole unpainted side must have received some light, 
whereas the opposite and painted side received none ; 
but a narrow zone on the unpainted side directly in 
front of the window will have received most light, and 
all the hinder parts (half an oval in section) less and less 
light in varying degrees ; and we may conclude that the 
angle of deflection is the resultant of the action of the 
light over the whole of the unpainted side. 

It should have been premised that painting with In- 
dian ink does not injure plants, at least within several 
hours ; and it could injure them only by stopping respi- 
ration. To ascertain whether injury was thus soon 
caused, the upper halves of 8 cotyledons of Avena were 
thickly coated with traTisparent matter — 4 with gum, and 
4 with gelatine ; they were placed in the morning before 
a window, and by the evening they were normally bowed 
towards the light, although the coatings now consisted 
of dry crusts of gum and gelatine. Moreover, if the 



4Y2 TRANSMITTED EFFECTS OF LIGHT. Chap. IX. 

seedlings whicli were painted longitudinally with Indian 
ink had been injured on the painted side, the opposite 
side would have gone on growing, and they would con- 
sequently have become bowed towards the painted side ; 
whereas the curvature was ahvays, as we have seen, in 
the opposite direction, or towards the unpainted side 
which was exposed to the light. We witnessed the 
effects of injuring longitudinally one side of the cotyle- 
dons of Avena and Phalaris ; for before we knew that 
grease was highly injurious to them, several were painted 
down one side with a mixture of oil and lamp-black, and 
were then exposed before a window ; others similarly 
treated were afterwards tried in darkness. These cotyle- 
dons soon became plainly bowed towards the blackened 
side, evidently owing to the grease on this side having 
checked their growth, whilst growth continued on the 
opposite side. But it deserves notice that the curvature 
differed from that caused by light, which ultimately 
becomes abrupt near the ground. These seedlings did 
not afterwards die, but were much injured and grew 
badly. 

Localised Sensitiyekess to Light, akd its 
transmitted effects. 

Phalaris Canariensis. — Whilst observing the accuracy 
with which the cotyledons of this plant became bent 
towards the light of a small lamp, we were impressed 
with the idea that the uppermost part determined the 
direction of the curvature of the lower part. When the 
cotyledons are exposed to a lateral light, the upper part 
bends first, and afterwards the bending gradually extends 
down to the base, and, as we shall presently see, even a 
little beneath the ground. This holds good with cotyle- 
dons from less than -1 inch (one was observed to act 
in this manner which was only -03 in height) to about -5 



Chap. IX. TRANSMITTED EFFECTS OF LIGHT. 473 

of an inch in height; but when they have grown to 
nearly an inch in height, the basal part, for a length of 
•15 to -2 of an inch above the ground, ceases to bend. 
As with young cotyledons the lower part goes on bend- 
ing, after the upper part has become well arched towards 
a lateral light, the apex would ultimately point to the 
ground instead of to the light, did not the upper part 
reverse its curvature and straighten itself, as soon as the 
upper convex surface of the bowed-down portion received 
more light than the lower concave surface. The position 

Fig. 181. 






Phalaris Canariensis : cotyledons after exposure in a box open on one 
side in front of a south-west window during 8 h. Curvature 
towards the light accurately traced. The short horizontal lines 
show the level of the ground. 

ultimately assumed by young and upright cotyledons, 
exposed to light entering obliquely from above through 
a window, is shown in the accompanying figure (Fig. 
181) ; and here it may be seen that the whole upper part 
has become very nearly straight. When the cotyledons 
were exposed before a very bright lamp, standing on the 
same level with them, the upper part, which was at first 
greatly arched towards the light, became straight and 
strictly parallel with the surface of the soil in the pots ; 
the basal part being now rectangularly bent. All this 
great amount of curvature, together with the subsequent 
straightening of the upper part, was often effected in a 
few hours. 

After the uppermost part has become bowed a little to the 
light, its overhanging weight must tend to increase the curva- 
ture of the lower part; but any such effect was shown in sevcrnl 
ways to be quite insignificant. Wlien little caps of tin-foil 



474 TRANSMITTED EFFECTS OF LIGHT. Chap. IX. 

(hereafter to be described) were placed on the summits of the 
cotyledons, though this must have added considerably to their 
weight, the rate or amount of bending was not thus increased. 
But the best evidence was afforded by placing pots with seed- 
lings of Phalaris before a lamp in such a position, that the coty- 
ledons were horizontally extended and projected at right angles 
to the line of light. In the course of 5| h. they were directed 
towards the light with their bases bent at right angles; and this 
abrupt curvature could not have been aided in the least by the 
weight of the upper part, which acted at right angles to the 
plane of curvature. 

It will be shown that when the upper halves of the coty- 
ledons of Phalaris and Avena were enclosed in little pipes of 
tin-foil or of blackened glass, in which case the upper part was 
mechanically prevented from bending, the lower and unen- 
closed part did not bend when exposed to a lateral light ; and 
it occurred to us that this fact might be due, not to the exclu- 
sion of the light from the upper part, but to some necessity of 
the bending gradually travelling down the cotyledons, so that 
unless the upper part first became bent, the lower could not 
bend, however much it might be stimulated. It was necessary 
for our purpose to ascertain whether this notion was true, and 
it was proved false; for the lower halves of several cotyledons 
became bowed to the light, although their upper halves were 
enclosed in little glass tubes (not blackened), which prevented, 
as far as we could judge, their bending. Nevertheless, as the 
part within the tube might possibly bend a very little, fine rigid 
rods or flat splinters of thin glass were cemented with shellac 
to one side of the upper part of 15 cotyledons; and in six cases 
they were in addition tied on with threads. They were thus 
forced to remain quite straight. The result was that the lower 
halves of all became bowed to the light, but generally not in so 
great a degree as the corresponding part of the free seedlings 
in the same pots; and this may perhaps be accounted for by 
some slight degree of injury having been caused by a consider- 
able surface having been smeared with shellac. It may be 
added; that when the cotyledons of Phalaris and Avena are 
acted on by apogeotropism, it is the upper part which begins 
first to bend; and when this part was rendered rigid in the 



Chap. IX. TRANSMITTED EFFECTS OF LIGHT. 475 

manner just described, the upward curvature of the basal part 
was not thus prevented. 

To test our belief that the upper part of the cotyledons of 
Phalaris, when exposed to a lateral light, regulates the bending 
of the lower part, many experiments were tried; but most of 
our first attempts proved useless from various causes not worth 
specifying. Seven cotyledons had their tips cut off for lengths 
varying between -1 and "IG of an inch, and these, when left 
exposed all day to a lateral light, remained upright. In an- 
other set of 7 cotyledons, the tips were cut off for a length of 
only about -05 of an inch (1-27 mm.) and these became bowed 
towards a lateral light, but not nearly so much as the many 
other seedlings in the same pots. This latter case shows that 
cutting off the tips does not by itself injure the plants so seri- 
ously as to prevent heliotropism ; but we thought at the time, 
that such injury might follow when a greater length was cut 
off, as in the first set of experiments. Therefore, no more trials 
of this kind were made, which we now regret; as we afterwards 
found that when the tips of three cotyledons were cut off for a 
length of '2 inch, and of four others for lengths of '14, '13, -1, 
and '07 inch, and they were extended horizontally, the amputa- 
tion did not interfere in the least with their bending vertically 
upwards, through the action of apogeotropism, like unmutilated 
specimens. It is therefore extremely improbable that the ampu- 
tation of the tips for lengths of from -1 to -14 inch, could from 
the injury thus caused have prevented the lower part from 
bending towards the light. 

We next tried the effects of covering the upper part of the 
cotyledons of Phalaris with little caps which were impermeable 
to light; the whole lower part being left fully exposed before a 
south-west window or a bright paraffin lamp. Some of the 
caps were made of extremely thin tin-foil blackened within; 
these had the disadvantage of occasionally, though rarely, being 
too heavy, especially when twice folded. The basal edges could 
be pressed into close contact with the cotyledons: though this 
again required care to prevent injuring them. Nevertheless, 
any injury thus caused could be detected by removing the caps, 
and trying whether the cotyledons were then sensitive to light. 
Other caps were made of tubes of the thinnest glass, which 



476 TRANSMITTED EFFECTS OF LIGHT. Chap. IX. 

when painted black served well, with the one great disadvan- 
tage that the lower ends could not be closed. But tubes were 
used which fitted the cotyledons almost closely, and black 
paper was placed on the soil round each, to check the upward 
reflection of light from the soil. Such tubes were in one respect 
far better than caps of tin-foil, as it was possible to cover at the 
same time some cotyledons with transparent and others with 
opaque tubes; and thus our experiments could be controlled. 
It should be kept in mind that young cotyledons were selected 
for trial, and that these when not interfered with become bowed 
down to the ground towards the light. 

We will begin with the glass-tubes. The summits of nine 
cotyledons, differing somewhat in height, were enclosed for 
rather less than half their lengths in uncoloured or transparent 
tubes; and these were then exposed before a south-west window 
on a bright day for 8 h. All of them became strongly curved 
towards the light, in the same degree as the many other free 
seedlings in the same pots; so that the glass-tubes certainly did 
not prevent the cotyledons from bending towards the light. 
Nineteen other cotyledons were, at the same time, similarly 
enclosed in tubes thickly painted with Indian ink. On five of 
them, the paint, to our surprise, contracted after exposure to 
the sunlight, and very narrow cracks were formed, through 
which a little light entered; and these five cases were rejected. 
Of the remaining 14 cotyledons, the lower halves of which had 
been fully exposed to the light for the whole time, 7 continued 
quite straight and upright; 1 was considerably bowed to the 
light, and 6 were slightly bowed, but with the exposed bases 
of most of them almost or quite straight. It is possible that 
some light may have been reflected upwards from the soil and 
entered the bases of these 7 tubes, as the sun shone brightly, 
though bits of blackened paper had been placed on the soil 
round them. Nevertheless, the 7 cotyledons which were slight- 
ly bowed, together with the 7 upright ones, presented a most 
remarkable contrast in appearance with the many other seed- 
lings in the same pots to which nothing had been done. The 
blackened tubes were then removed from 10 of these seedlings, 
and they were now exposed before a lamp for 8 h. : 9 of them 
became greatly, and 1 moderately, curved towards the light. 



Chap. IX. TRANSMITTED EFFECTS OF LIGHT. 47 Y 

proving that the previous absence of any curvature in the basal 
part, or the presence of only a slight degree of curvature there, 
was due to the exclusion of light from the upper part. 

Similar observations were made on 12 younger cotyledons 
with their upper halves enclosed within glass tubes coated with 
black varnish, and with their lower halves fully exposed to 
bright sunshine. In these younger seedlings the sensitive zone 
seems to extend rather lower down, as was observed on some 
other occasions, for two became almost as much curved towards 
the light as the free seedlings; and the remaining ten were 
slightly curved, although the basal part of several of them, 
which normally becomes more curved than any other part, ex- 
hibited hardly a trace of curvature. These 13 seedlings taken 
together differed greatly in their degree of curvature from all 
the many other seedlings in the same pots. 

Better evidence of the efficiency of the blackened tubes was 
incidentally afforded ])y some experiments hereafter to be given,- 
in which the upper halves of 14 cotyledons were enclosed in 
tubes from which an extremely narrow stripe of the black var- 
nish had been scraped off. These cleared stripes were not 
directed towards the window, but obliquely to one side of the 
room, so that only a very little light could act on the upper 
halves of the cotyledons. These 14 seedlings remained during 
eight hours of exposure before a south-west window on a hazy 
day quite upright; whereas all the other many free seedlings in 
the same pots became greatly bowed towards the light. 

We will now turn to the trials with caps made of very thin 
tin-foil. These were placed at different times on the summits of 
24 cotyledons, and they extended down for a length of between 
•15 and '2 of an inch. The seedlings were exposed to a lateral 
light for periods varying between 6 h. 30 m. and 7 h. 45 m., 
which sufficed to cause all the other seedlings in the same i)ots 
to become almost rectangularly bent towards the light. They 
varied in height from only -04 to 1*15 inch, but the greater 
number were about •'fS inch. Of the 24 cotyledons with their 
summits thus protected, 3 became much bent, but not in the 
direction of the light, and as they did not straighten themselves 
through apogeotropism during the following night, either tlie 
caps were too heavy or the plants themselves were in a weak 



478 TRANSMITTED EFFECTS OF LIGHT. Chap. IX. 

condition; and these three cases may be excluded. There 
are left for consideration 21 cotyledons; of these 17 remained 
all the time quite upright ; the other 4 became slightly inclined 
to the light, but not in a degree comparable with that of the 
many free seedlings in the same pots As the glass-tubes, when 
unpainted, did not prevent the cotyledons from becoming 
greatly bowed, it cannot be supposed that the caps of very 
thin tin-foil did so, except through the exclusion of the light. 
To prove that the plants had not been injured, the caps were 
removed from 6 of the upright seedlings, and these were exposed 
before a paraffin lamp for the same length of time as before, 
and they now all became greatly curved towards the light. 

As caps between -15 and '2 of an inch in depth were thus 
proved to be highly efficient in preventing the cotyledons from 
bending towards the light, 8 other cotyledons were protected 
with caps between only '06 and '12 in depth. Of these, two 
remained vertical, one was considerably and five slightly curved 
towards the light, but far less so than the free seedlings in the 
same pots. 

Another trial was made in a different manner, namely, by 
bandaging with strips of tin-foil, about -02 in breadth, the upper 
part, but not the actual summit, of eight moderately young 
seedlings a little over half an inch in height. The summits and 
the basal parts were thus left fully exposed to a lateral light 
during 8 h. ; an upper intermediate zone being protected. 
With four of these seedlings the summits were exposed for 
a length of -05 inch, and in two of them this part became 
curved towards the light, but the whole lower part remained 
quite upright; whereas the entire length of the other two 
seedlings became slightly curved towards the light. The 
summits of the four other seedlings were exposed for a length 
of -04 inch, and of these one remained almost upright, whilst 
the other three became considerably curved towards the light. 
The many free seedlings in the same pots were all greatly 
curved towards the light. 

From these several sets of experiments, including those with 
the glass-tubes, and those when the tips were cut off, we may 
infer that the exclusion of light from the upper part of the 
cotyledons of Phalaris prevents the lower part, though fully 



Chap. IX. TRANSMITTED EFFECTS OF LIGHT. 479 

exposed to a lateral light, from becoming curved. The summit 
for a length of -04 or -05 of an inch, though it is itself sensitive 
and curves tov^ards the light, has only a slight power of causing 
the lower part to bend. Nor has the exclusion of light from the 
summit for a length of '1 of an inch a strong influence on the 
curvature of the lower part. On the other hand, an exclusion 
for a length of between -15 and 2 of an inch, or of the whole 
upper half, plainly prevents the lower and fully illuminated 
part from becoming curved in the manner (see Fig. 181) which 
invariably occurs when a free cotyledon is exposed to a lateral 
light. With very young seedlings the sensitive zone seems to 
extend rather lower down relatively to their height than in older 
seedlings. We must therefore conclude that when seedlings 
are freely exposed to a lateral light some influence is trans- 
mitted from the upper to the lower part, causing the latter to 
bend. 

This conclusion is supported by what may be seen to occur 
on a small scale, especially with young cotyledons, without any 
artificial exclusion of the light; for they bend beneath the earth 
where no light can enter. Seeds of Phalaris were covered 
with a layer one fourth of an inch in thickness of very fine 
sand, consisting of extremely minute grains of silex coated with 
oxide of iron. A layer of this sand, moistened to the same 
degree as that over the seeds, w^as spread over a glass-plate ; and 
when the layer was -05 of an inch in thickness (carefully meas- 
ured) no light from a bright sky could be seen to pass through 
it, unless it was viewed through a long blackened tube, and 
then a trace of light could be detected, but probably mucli too 
little to affect any plant. A layer -1 of an inch in thickness was 
quite impermeable to light, as judged by the eye aided by the 
tube. It may be worth adding that the layer, when dried, re- 
mained equally impermeable to light. This sand yielded to very 
slight pressure whilst kept moist, and in this state did not con- 
tract or crack in the least. In a first trial, cotyledons which 
had grown to a moderate height were exposed for 8 h. before 
a paraffin lamp, and they became greatly bowed. At their 
bases on the shaded side opposite to the light, well-defined, 
crescentic, open furrows were formed, which (measured under 
a microscope with a micrometer) were from "02 to -03 of an 



480 TRANSMITTED EFFECTS OF LIGHT. Chap. IX. 

inch in breadth, and these had evidently been left by the 
bending of the buried bases of the cotyledons towards the 
light. On the side of the light the cotyledons were in close 
contact with the sand, which was a very little heaped up. By 
removing with a sharp knife the sand on one side of the coty- 
ledons in the line of the light, the bent portion and the open 
furrows vrere found to extend down to a depth of about '1 of 
an inch, where no light could enter. The chords of the short 
buried arcs formed in four cases angles of 11°, 13°, 15°, and 
18", with the perpendicular. By the following morning these 
short bowed portions had straightened themselves through 
apogeotropism. 

In the next trial much younger cotyledons were similarly 
treated, but were exposed to a rather obscure lateral light. 
After some hours, a bowed cotyledon, -3 inch in height, had an 
open furrow on the shaded side '04 inch in breadth ; another 
cotyledon, only -13 inch in height, had left a furrow "02 inch in 
breadth. But the most curious case was that of a cotyledon which 
had just protruded above the ground and was only '03 inch in 
height, and this was found to be bowed in the direction of the 
light to a depth of '2 of an inch beneath the surface. From 
what we know of the impermeability of this sand to light, the 
upper illuminated part in these several cases must have deter- 
mined the curvature of the lower buried portions. But an 
apparent cause of doubt may be suggested : as the cotyledons 
are continually circumnutating, they tend to form a minute 
crack or furrow all round their bases, which would admit a 
little light on all sides; but this would not liappen when they 
were ilhiminated laterally, for w^e know^ that they quickly bend 
towards a lateral light, and they tlien press so firmly against the 
sand on the illuminated side as to furrow it, and this would 
etfectually exclude light on this side. Any light admitted on 
the opposite and shaded side, where an open furrow is formed, 
would tend to counteract the curvature tow\irds the lam]) or 
other source of the light. It may be added, that tlie use of 
fine moist sand, which yields easily to pressure, was indispen- 
sable in the above experiments-, for seedlings raised in com- 
mon soil, not kept especially damp, and exposed for 9 h. 30 m. 
to a strong lateral light, did not form an open furrow at their 



Chap. IX. TEANSMITTED EFFECTS OF LIGHT, 481 

bases on the shaded side, and were not bowed beneath the 
surface. 

Perhaps the most striking proof of the action of the upper 
on the lower part of tlie cotyledons of Phalaris, when laterally 
illuminated, was afforded by the blackened glass-tubes (before 
alluded to) with very narrow stripes of the varnished scraped 
off on one side, througli which a little light was admitted. 
The breadth of these stripes or slits varied between -01 and 
"02 inch (-25 and '51 mm.). Cotyledons' with their upper 
halves enclosed in such tubes were placed before a south-west 
window, in such a position, that the scraped stripes did not 
directly face the window but obliquely to one side. The seed- 
lings w^ere left exposed for 8 h., before the close of which time 
the many free seedlings in the same pots had become greatly 
bowed towards the window. Under these circumstances, tlie 
whole lower halves of the cotyledons, which had their summits 
enclosed in the tubes, vv^ere fully exposed to the light of the 
sky, whilst their upper halves received exclusively or chiefly 
diffused light from the room, and this only through a very 
narrow slit on one side. Now, if the curvature of the lower 
part had been determined by the illumination of this part, all 
the cotyledons assuredly would have become curved towards 
the window; but this was far from being the case. Tubes 
of the kind just described were placed on several occ:isions 
over the upper halves of 27 cotyledons ; 14 of them remained 
all the time quite vertical; so that sufl[icient diffused light 
did not enter through the narrow slits to produce any effect 
whatever; and they behaved in the same manner as if their 
upper halves had been enclosed in completely blackened tubes. 
The lower halves of the 13 other cotyledons became bowed 
not directly in the line of the window, but obliquely towards 
it; one pointed at an angle of only 18°, but the remaining 12 
at angles varying between 45'' and 62° from tlie line of the 
window. At the commencement of the exjieriment, pins hud 
been laid on the earth in the direction towards which the slits 
in the varnish faced; and in this direction alone a small amount 
of diffused light entered. At the close of the experiment, 7 of 
the bowed cotyledons pointed exactly in the line of tlie ])ins, 
and G of them in a line between that of the pins and that of the 



482 TRANSMITTED EFFECTS OF LIGHT. Chap. IX. 

window. This intermediate position is intelligible, for any light 
from the sky which entered obliquely through the slits would 
be much more efficient than the diffused light which entered 
directly through them. After the 8 h. exposure, the contrast 
in appearance between these 13 cotyledons and the many other 
seedlings in the same pots, which were all (excepting the above 
14 vertical ones) greatly bowed in straight and parallel lines 
towards the window, was extremely remarkable. It is therefore 
certain that a little weak light striking the upper halves of the 
cotyledons of Phalaris, is far more potent in determining the 
direction of the curvature of the lower halves, than the full 
illumination of the latter during the whole time of exposure. 

In confirmation of the above results, the effect of thickly 
painting with Indian ink one side of the upper part of three coty- 
ledons of Phalaris, for a length of '2 inch from their tips, may be 
worth giving. These were placed so that the unpainted surface 
was directed not towards the window, but a little to one side ; 
and they all became bent towards the unpainted side, and from 
the line of the window by angles amounting to 31°, 35°, and 83°. 
The curvature in this direction extended down to their bases, 
although the whole lower part was fully exposed to the light 
from the window. 

Finally, although there can be no doubt that the illumination 
of the upper part of the cotyledons of Phalaris greatly affects 
the power and manner of bending of the lower part, yet some 
observations seemed to render it probable that the simultaneous 
stimulation of the lower part by light greatly favours, or is 
almost necessary, for its well-marked curvature; but our experi- 
ments were not conclusive, owing to the difficulty of excluding 
light from the lower halves without mechanically preventing 
their curvature. 

Avena sativa.— The cotyledons of this plant become quickly 
bowed towards a lateral light, exactly like those of Phalaris. 
Experiments similar to the foregoing ones were tried, and we 
will give the results as briefly as possible. They are somewhat 
less conclusive than in the case of Phalaris, and this may 
possibly be accounted for by the sensitive zone varying in exten- 
sion, in a species so long cultivated and variable as the common 
Oat. Cotyledons a little under three-quarters of an inch in 



Chap. IX. TRANSMITTED EFFECTS OF LIGHT. 483 

heig-ht were selected for trial: six had their summits protected 
from light by tin-foil caps, '25 inch in depth, and two others by 
caps '3 inch in depth. Of these 8 cotyledons, five remained 
upright during 8 hours of exposure, although their lower parts 
were fully exposed to the light all the time ; two were very 
slightly and one considerably, bowed towards it. Caps only -2 
or '22 inch in depth were placed over 4 other cotyledons, and 
now only one remained upright, one was slightly, and two 
considerably bowed to the light. In this and the following cases 
all the free seedlings in the same pots became greatly bowed to 
the light. 

Our next trial was made with short lengths of thin and 
fairly transparent quills ; for glass-tubes of sufficient diameter to 
go over the cotyledons would have been too heavy. Firstly, 
the summits of 13 cotyledons were enclosed in unpainted quills, 
and of these 11 became greatly and 2 slightly bowed to the 
light; so that the mere act of enclosure did not prevent the 
lower part from becoming bowed. Secondly, the summits of 
11 cotyledons were enclosed in quills -3 inch in length, painted 
so as to be impermeable to light ; of these, 7 did not become 
at all inclined towards the light, but 3 of them were slightly 
bent more or less transversely with respect to the line of 
light, and these might perhaps have been altogether excluded ; 
one alone was slightly bowed towards tlie light. Painted 
quills, -25 inch in length, were placed over the summits of 4 
other cotyledons; of tliese, one alone remained upright, a 
second was slightly bowed, and the two others as much bowed 
to the light as the free seedlings in the same pots. These two 
latter cases, considering that the caps were -25 in length, are 
inexplicable. 

Lastly, the summits of 8 cotyledons were coated with flexible 
and highly transparent gold-beaters' skin, and all be(!ame as 
much bowed to the light as the free seedlings. The summits of 
9 other cotyledons were similarly coated with gold-beaters' skin, 
which was then painted to a depth of between -25 and -3 inch, 
so as to be impermeable to light ; of these 5 remained upright, 
and 4 were well bowed to the light, almost or quite as well as 
the free seedlings. These latter four cases, as well as the two 
in the last paragraph, offer a strong exception to the rule that 



484 TRANSMITTED EFFECTS OF LIGHT. Chap. IX. 

the illumination of the upper part determines the curvature of 
the lower part. Nevertheless, 5 of these 8 cotyledons remained 
quite upright, although their lower halves were fully illumi- 
nated all the time ; and it would almost be a prodigy to find 
five free seedlings standing vertically after an exposure for sev- 
eral hours to a lateral light. 

The cotyledons of Avena, like those of Phalaris, when 
growing in soft, damp, fine sand, leave an open crescentic fur- 
row on tlie shaded side, after bending to a lateral light ; and 
they become bowed beneath the surface at a depth to which, as 
we know, light cannot penetrate. The arcs of the chords of the 
buried bowed portions formed in two cases angles of 20° and 
21° with the perpendicular. The open furrows on the shaded 
side were, in four cases, -008, -016, -024, and "024 of an inch in 
breadth. 

Brassica oleracea (Common Red). — It will here be shown that 
the upper half of the hypocotyl of the cabbage, when illuminated 
by a lateral light, determines the curvature of the lower half. 
It is necessary to experimentise on young seedlings about half 
an inch or rather less in height, for when grown to an inch and 
upwards the basal part ceases to bend. We first tried painting 
the hypocotyls with Indian ink, or cutting off their summits for 
various lengths; but these experiments are not worth giving, 
though they confirm, as far as they can be trusted, the results 
of the following ones. These were made by folding gold-beat- 
ers' skin once round the upper halves of young hypocotyls, and 
painting it thickly with Indian ink or with black grease. As 
a control experiment, the same transparent skin, left unpainted, 
was folded round the upper halves of 12 hypocotyls: and these 
all became greatly curved to the light, excepting one which was 
only moderately curved. Twenty other young hypocotyls had 
the skin round their upper halves painted, whilst their lower 
halves were left quite uncovered. These seedlings were then 
exposed, generally for between 7 and 8 h., in a box blackened 
within and open in front, either before a south-west window or 
a paraffin lamp. This exposure was amply sufficient, as was 
shown by the strongly-marked heliotropism of all the free seed- 
lings in the same pots; nevertheless, some were left exposed 
to the light for a much longer time. Of the 20 hypocotyls 



Chap. IX. TRANSMITTED EFFECTS OF LIGHT. 485 

thus treated, 14 remained quite upright, and 6 became slightly- 
bowed to the light ; but 2 of these latter cases were not really 
exceptions, for on . removing the skin the paint was found im- 
perfect and was penetrated by many small transparent spaces 
on the side which faced the light. Moreover, in two other cases 
the painted skin did not extend quite halfway down the hypo- 
cotyl. Altogether there was a wonderful contrast in the several 
pots between these 20 hypocotyls and the other many free 
seedlings, which were all greatly bowed down to their bases in 
the direction of the light, some being almost prostrate on the 
ground. 

The most successful trial on any one day (included in the 
above results) is worth describing in detail. Six young seed- 
lings were selected, the hypocotyls of which were nearly -45 
inch, excepting one, which w^as -6 inch in height, measured from 
the bases of their petioles to the ground. Their upper halves, 
judged as accurately as could be done by the eye, were folded 
once round with gold-beaters' skin, and this was painted 
thickly with Indian ink. They were exposed in an otherwise 
darkened room before a bright paraffin lamp, which stood on 
a level with the two pots containing the seedlings. Tliey 
were first looked at after an interval of 5 h. 10 m., and five 
of the protected hypocotyls were found quite erect, the sixth 
being very slightly inclined to the light ; whereas all the many 
free seedlings in the same two pots were greatly bowed to the 
light. They were again examined after a continuous exposure 
to the light of 20 h. 35 m. ; and now the contrast between the 
two sets was wonderfully great ; for the free seedlings had their 
hypocotyls extended almost horizontally in the direction of the 
light, and were curved down to the ground ; wiiilst those with 
the upper halves protected by the painted skin, but with their 
lower halves fully exposed to the light, still remained quite 
upright, with the exception of the one which retained the same 
slight inclination to the light which it had before. This latter 
seedling was found to have been rather hndly painted, for on 
the side facing the light the red colour of the hypocotyl could 
be distinguished through the paint. 

We next tried nine older seedlings, the hypocotyls of wliicli 
varied between 1 and 1-G inch in height. The gold-beaters' 



486 TRANSMITTED EFFECTS OF LIGHT. Chap. IX. 

skin round their upper parts was painted witli black grease to 
a depth of only '3 inch, that is, from less than a third to a fourth 
or fifth of their total heights. They were exposed to the light 
for 7 h. 15 m. ; and the result showed that the whole of the 
sensitive zone, which determines the curvature of the lower 
part, was not protected from the action of the light ; for all 9 
became curved towards it, 4 of them very slightly, 3 moderately, 
and 2 almost as much as the unprotected seedlings, Neverthe- 
less, the whole 9 taken together differed plainly in their degree 
of curvature from the many free seedlings, and from some 
which were wrapped in unpainted skin, growing in the same 
two pots. 

Seeds were covered with about a quarter of an inch of the fine 
sand described under Phalaris ; and when the hypocotyls had 
grown to a height of between -4 and '55 inch, they were exposed 
during 9 h. before a paraffin lamp, their bases being at first 
closely surrounded by the damp sand. They all became bowed 
down to the ground, so that their upper parts lay near to and 
almost parallel to the surface of the soil. On the side of the 
light their bases were in close contact with the sand, which was 
here a very little heaped up; on the opposite or shaded side 
there were open, crescentic cracks or furrows, rather above "01 
of an inch in width ; but they were not so sharp and regular 
as those made by Phalaris and Avena, and therefore could not 
be so easily measured under the microscope. The hypocotyls 
were found, when the sand was removed on one side, to be 
curved to a depth beneath the surface in three cases of at least 
•1 inch, in a fourth case of -11, and in a fifth of -15 inch. The 
chords of the arcs of the short, buried, bowed portions formed 
angles of between 11° and 15° with the perpendicular. From 
what we have seen of the impermeability of this sand to light, 
the curvature of the hypocotyls certainly extended down to a 
depth where no light could enter; and the curvature must 
have been caused by an influence transmitted from the upper 
illuminated part. 

The lower halves of five young hypocotyls were surrounded 
by unpainted gold-beaters' skin, and these, after an exposure 
of 8 h. before a paraffin lamp, all became as much bowed to the 
light as the free seedlings. The lower halves of 10 other young 



CHAP..IX. TRANSMITTED EFFECTS OF LIGHT. 487 

hypocotj'ls, similarly surrounded with the skin, were thickly 
painted with Indian ink; their upper and unprotected halves 
became well curved to the light, but their lower and protected 
halves remained vertical in all the cases excepting one, and on 
this the layer of paint was imperfect. This result seems to 
prove that the influence transmitted from the upper part is not 
sufficient to cause the lower part to bend, unless it be at the 
same time illuminated ; but there remains the doubt, as in the 
case of Phalaris, whether the skin covered with a rather thick 
crust of dry Indian ink did not mechanically prevent their 
curvature. 

Beta vulgaris. — A few analogous experiments were tried on 
this plant, which is not very well adapted for the purpose, as 
th« basal part of the hypocotyl, after it has grown to above half 
an inch in height, does not bend much on exposure to a lateral 
light. Four hypocotyls were surrounded close beneath their 
petioles with strips of thin tin-foil, '2 inch in breadth, and they 
remained upright all day before a paraffin lamp; two others 
were surrounded with strips -15 inch in breadth, and one of 
these remained upright, the other becoming bowed; the band- 
ages in two other cases were only '1 inch in breadth, and both 
of these hypocotyls became bowed, though one only slightly, 
towards the light. The free seedlings in the same pots were 
all fairly well curved towards the light ; and during the follow- 
ing night became nearly upright. The pots were now turned 
round and placed before a window, so that the opposite sides 
of the seedlings were exposed to the light, towards which all 
the unprotected hypocotyls became bent in the course of 7 h. 
Seven out of the 8 seedlings with bandages of tin-foil remained 
upright, but one which had a bandage only -1 inch in breadth, 
became curved to the light. On another occasion, the upper 
halves of 7 hypocotyls were surrounded with painted gold- 
beaters' skin; of those 4 remained upright, and 3 became a little 
curved to the light: at the same time 4 other seedlings sur- 
rounded with unpainted skin, as well as the free ones in the 
same pots, all became bowed towards the lamp, before which 
they had been exposed during 22 hours. 

Radicles of Sinapis aTba.— The radicles of some plants are 
indifferent, as far as curvature is concerned, to the action of 
32 



488 TRANSMITTED EFFECTS OF LIGHT. Chap. IX. 

light ; whilst others bend towards and others from it.* Whether 
these movements are of any service to the plant is very doubtful, 
at least in the case of subterranean roots ; they probably result 
from the radicles being sensitive to contact, moisture, and gravi- 
tation, and as a consequence to other irritants which are never 
naturally encountered. The radicles of Sinapis alba, when 
immersed in water and exposed to a lateral light, bend from it, 
or are apheliotropic. They become bent for a length of about 
4 mm. from their tips. To ascertain whether this movement 
generally occurred, 41 radicles, which had germinated in damp 
sawdust, were immersed in water and exposed to a lateral light ; 
and they all, with two doubtful exceptions, became curved from 
the light. At the same time the tips of 54 other radicles, 
similarly exposed, were just touched with nitrate of sil\^r. 
They were blackened for a length of from '05 to "O? mm., and 
probably killed ; but it should be observed that this did not 
check materially, if at all, the growth of the upper part; for 
several, which were measured, increased in the course of only 
8-9 h. by 5 to 7 mm, in length. Of the 54 cauterised radicles 
one case was doubtful, 25 curved themselves from the light in 
the normal manner, and 28, or more than half, were not in the 
least apheliotropic. There was a considerable difference, which 
we cannot account for, in the results of the experiments tried 
towards the end of April and in the middle of September, 
Fifteen radicles (part of the above 54) were cauterised at the 
former period and were exposed to sunshine, of which 12 failed 
to be apheliotropic, 2 were still apheliotropic, and 1 was doubt- 
ful. In September, 39 cauterised radicles were exposed to a 
northern light, being kept at a proper temperature; and now 
23 continued to be apheliotropic in the normal manner, and 
only 16 failed to bend from the light. Looking at the aggregate 
results at both periods, there can be no doubt that the destruc- 
tion of the tip for less than a millimeter in length destroyed 
in more than half the cases their power of moving from the 
light. It is probable that if the tips had been cauterised for 
the length of a whole millimeter, all signs of apheliotropism 
would have disappeared. It may be suggested that although 



Sachs, 'Physiologic Vegetale,' 1868, p. 44, 



Chap. IX. COXCLUDING REMARKS. 489 

the application of caustic does not stop growth, yet enough may 
be absorbed to destroy the power of movement in the upper 
part; but this suggestion must be rejected, for we have seen and 
shall again see, that cauterising one side of the tip of various 
kinds of radicles actually excites movement. The conclusion 
seems inevitable that sensitiveness to light resides in the tip 
of the radicle of Sinapis alba; and that the tip when thus 
stimulated transmits some influence to the upper part, causing 
it to bend. The case in this respect is parallel with that of 
the radicles of several plants, the tips of which are sensitive to 
contact and to other irritants, and, as will be shown in the 
eleventh chapter, to gravitation. 

CoxcLUDii^G Eemarks an^d Summart of Chapter. 

We do not know whether it is a general rule with 
seedling plants that the illumination of the upper part 
determines the curvature of the lower part. But as this 
occurred in the four species examined by us, belonging 
to such distinct families as the Gramineae, Cruciferae, 
and Chenopodeae, it is probably of common occurrence. 
It can hardly fail to be of service to seedlings, by aiding 
them to find the shortest path from the buried seed to 
the light, on nearly the same principle that the eyes of 
most of the lower crawling animals are seated at the 
anterior ends of their bodies. It is extremely doubtful 
whether with fully developed plants the illumination of 
one part ever affects the curvature of another part. The 
summits of 5 young plants of A.^paragus officinalis (vary- 
ing in height between 1-1 and 2-7 inches, and consisting 
of several short internodes) were covered with caps of 
tin-foil from 0-3 to 0-35 inch in depth ; and the lower 
uncovered parts became as much curved to\vard a lat- 
eral light, as were the free seedlings in the same pots. 
Other seedlings of the same plant had their summits 
painted with Indian ink with the same negative result. 
Pieces of blackened paper were gummed to the edges 



490 CONCLUDING REMARKS AND Chap. IX. 

and over the blades of some leaves on young plants of 
TropcBoliim majus and Ranunculus ficaria ; these were 
then placed in a box before a window, and the petioles 
of the protected leaves became curved towards the light, 
as much as those of the unprotected leaves. 

The foregoing cases with respect to seedling plants 
have been fully described, not only because the trans- 
mission of any effect from light is a new physiological 
fact, but because we think it tends to modify somewhat 
the current views on heliotropic movements. Until 
lately such movements were believed to result simply 
from increased growth on the shaded side. At present 
it is commonly admitted * that diminished light increases 
the turgescence of the cells, or the extensibility of the 
cell- walls, or of both together, on the shaded side, and 
that this is followed by increased growth. But Pfeffer 
has shown that a difference in the turgescence on the two 
sides of a pulvinus, — that is, an aggregate of small cells 
which have ceased to grow at an early age, — is excited 
by a difference in the amount of light received by the 
two sides ; and that movement is thus caused without 
being followed by increased growth on the more turges- 
cent side.f All observers apparently believe that light 
acts directly on the part which bends, but we have seen 
with the above described seedlings that this is not the 
case. Their lower halves were brightly illuminated for 



"'■• Emil Godlewski has given 63, 123, &c. Frank has also 

('Bot. Zeitung,' 1879, Nos. 6-9) insisted ('Die Natiirliche wa- 

an excellent account (p. 120) of gerechte Eichtung von Pflan- 

the present state of the question, zentheilen,' 1870, p. 53) on the 

See also Vines in ' Arbeiten des important part which the pulvini 

Bot. Inst, in Wiirzburg,' 1878, B. of the leaflets of compound leaves 

ii. pp. 114-147. Hugo de Vries play in placing the leaflets in a 

has recently published a still proper position with respect to the 

more important article on this light. This holds good, especially 

subject : ' Bot. Zeitung,' Dec. 19th with the leaves of climbing plants, 

and 26th, 1879. which are carried into all sorts 

t ' Die Periodischen Bewegun- of positions, ill-adapted for the 

gen der Blattorgane,' 1875, pp. 7, action of the light. 



Chap. IX. SUMMARY OF CHAPTER. 491 

hours, and yet did not bend in the least toward^the light, 
though tliis is the part which under ordinary circum- 
stances bends the most. It is a still more striking fact, 
that the faint illumination of a narrow stripe on one 
side of the upper part of the cotyledons of Phalaris 
determined the direction of the curvature of the lower 
part ; so that this latter part did not bend towards the 
bright light by which it had been fully illuminated, but 
obliquely towards one side where only a little light en- 
tered. These results seem to imply the presence of 
some matter in the upper part which is acted on by 
light, and which transmits its effects to the lower part. 
It has been shown that this transmission is independ- 
ent of the bending of the upper sensitive part. We 
have an analogous case of transmission in Drosera, for 
when a gland is irritated, the basal and not the upper 
or intermediate part of the tentacle bends. The flexi- 
ble and sensitive filament of Diona3a likewise transmits 
a stimulus, without itself bending ; as does the stem of 
Mimosa. 

Light exerts a powerful influence on most vegetable 
tissues, and there can be no doubt that it generally tends 
to check their growth. But when the two sides of a 
plant are illuminated in a slightly different degree, it 
does not necessarily follow that the bending towards the 
illuminated side is caused by changes in the tissues of 
the same nature as those which lead to increased growth 
in darkness. We know at least that a part may bend 
from the light, and yet its growth may not be favoured 
by light. This is the case with the radicles of Sinapis 
alba, which are plainly apheliotropic ; nevertheless, they 
grow quicker in darkness than in light.* So it is with 

* Francis Darwin, ' Ueber das Bot. Inst, in Wiirzburg,' IJ. ii., 
Wachsthum negativ heliotropi- Heft ill., 1880, p. 521. 
scher Wurzeln ' : ' Ar])eitcn dcs 



492 CONCLUDING REMARKS AND Chap. IX. 

many aerml roots, according to Wiesner ; * but there are 
other opposed cases. It appears, therefore, that light 
does not determine the growth of apheiiotropic parts in 
any uniform manner. 

We should bear in mind that the power of bending 
to the light is highly beneficial to most plants. There 
is therefore no improbability in this power having been 
specially acquired. In several respects light seems to 
act on plants in nearly the same manner as it does on 
animals by means of the nervous system. f With seed- 
lings the effect, as we have just seen, is transmitted from 
one part to another. An animal may be excited to 
move by a very small amount of light ; and it has been 
shown that a difference in the illumination of the two 
sides of the cotyledons of Phalaris, which could not be 
distinguished by the human eye, sufficed to cause them 
to bend. It has also been shown that there is no close 
parallelism between the amount of light which acts on a 
plant and its degree of curvature ; it was indeed hardly 
possible to perceive any difference in the curvature of 
some seedlings of Phalaris exposed to a light, w^hich 
though dim, was very much brighter than that to w^hich 
others had been exposed. The retina, after being stimu- 
lated by a bright light, feels the effect for some time ; 
and Phalaris continued to bend for nearly half an hour 
towards the side which had been illuminated. The 
retina cannot perceive a dim light after it has been ex- 
posed to a bright one ; and plants which had been kept 
in the daylight during the previous day and morning, 
did not move so soon towards an obscure lateral light as 
did others which had been kept in complete darkness. 



■'•• 'Sitzb. der k. Akad. der Wis- which excite movement in plants, 

sensch.' (Vienna). 1880, p. 12. See his paper ' Ueber orthotrope 

t Sachs has made some striking und plagiotrope Pflanzentheile, 

remarks to the same effect Avith ' Arb. des Bot. Inst, in Wiirzburg, 

respect to the various stimuli 1879, B. ii. p. 282, 



Chap. IX. SUMMARY OF CHAPTER. 493 

Even if light does act in such a manner on the grow- 
ing parts of plants as always to excite in them a tendency 
to bend towards the more illuminated side — a supposi- 
tion contradicted by the foregoing experiments on seed- 
lings and by all apheliotropic organs — yet the tendency 
differs greatly in different species, and is variable in de- 
gree in the individuals of the same species, as may be 
seen in almost any pot of seedlings of a long cultivated 
plant.* There is therefore a basis for the modification 
of this tendency to almost any beneficial extent. That 
it has been modified, we see in many cases : thus, it is of 
more importance for insectivorous plants to place their 
leaves in the best position for catching insects than to 
turn their leaves to the light, and they have no such 
power. If the stems of twining plants were to bend 
towards the light, they would often be drawn away from 
their supports ; and as we have seen they do not thus 
bend. As the stems of most other plants are heliotropic, 
we may feel almost sure that twining plants, which are 
distributed, throughout the whole vascular series, have 
lost a power that their non-climbing progenitors pos- 
sessed. Moreover, with Ipomoea, and probably all other 
twiners, the stem of the young plant, before it begins to 
twine, is highly heliotropic, evidently in order to expose 
the cotyledons or the first true leaves fully to the light. 



•-■• Strasburger has shown in his the light. Some individuals, more- 
interesting work ('Wirkung des over, appear to be indifferent to 
Lichtes . . . auf Schwiirmsporen,' the light; and those of different 
1878), that the movement of the species behave very differently, 
swarm-spores of various loAvly The brighter the light, the 
organised plants to a lateral light straighter is their course. They 
is influenced by their stage of exhibit also for a short time the 
development, by the temperature after-effects of light. In all these 
to which they are subjected, by respects they resemble the higher 
the degree of "illumination under plants. vSee, also, Stahl, ' Ueber 
which they have been raised, and den einfluss der Lichls auf die 
by other unknown causes ; so that Bewegungs - erscheinungen der 
the swtirm-spores of the same Schwiirmsporen' Verb. d. phys.- 
species may move across the field med. Gesellschaft in Wiirzburg, 
of the microscope either to or from B. xii. 1878. 



494 CONCLUDIXG REMARKS AND Chap. IX. 

With the Ivy the stems of seedlings are moderately heli- 
otropic, whilst those of the same plants when grown a 
little older are apheliotropic. Some tendrils which con- 
sist of modified leaves — organs in all ordinary cases 
strongly diaheliotropic — have been rendered aphelio- 
tropic, and their tips crawl into any dark crevice. 

Even in the case of ordinary helio tropic movements, 
it is hardly credible that they result directly from the 
action of the light, without any special adaptation. We 
may illustrate what we mean by the hygroscopic move- 
ments of plants : if the tissues on one side of an organ 
permit of rapid evaporation, they will dry quickly and 
contract, causing the part to bend to this side. ]S"ow the 
wonderfully complex movements of the pollinia of Orchis 
pyramiclalis, by which they clas]) the proboscis of a moth 
and afterwards change their position for the sake of de- 
positing the pollen- masses on the double stigma — or again 
the twisting movements, by which certain seeds bury 
themselves in the ground * — follow from the manner of 
drying of the parts in question ; yet no one will suj)pose 
that these results have been gained without special adap- 
tation. Similarly, we are led to believe in adaptation 
when we see the hypocotyl of a seedling, which contains 
chlorophyll, bending to the light ; for although it thus 
receives less light, being now shaded by its own cotyle- 
dons, it places them — the more important organs — in the 
best position to be fully illuminated. The hypocotyl 
m.ay therefore be said to sacrifice itself for the good of 
the cotyledons, or rather of the whole plant. But if it 
be prevented from bending, as must sometimes occur 
with seedlings springing up in an entangled mass of 
vegetation, the cotyledons themselves bend so as to face 
the light ; the one farthest off rising up, and that near- 

* Francis Darwin, ' On the Hy- actions Linn. Soc.,' series ii. vol. i. 
groscopicMecluinism,'&c., 'Trans- p. 149, 1876. 



Chap. IX. SUMMAPtY OF CHAPTER. 495 

est to the light sinking down, or both twisting laterally.* 
We may, also, suspect that the extreme sensitiveness to 
light of the upper part of the sheath-like cotyledons of 
the Graniineas, and their power of transmitting its effects 
to the lower part, are specialised arrangements for find- 
ing the shortest path to the light. With plants growing 
on a bank, or thrown prostrate by the wind, the manner 
in which the leaves move, even rotating on their own 
axes, so that their upper surfaces may be again directed 
to the light, is a striking phenomenon. Such facts are 
rendered more striking when we remember that too in- 
tense a light injures the chlorophyll, and that the leaf- 
lets of several Leguminosse when thus exposed bend 
upwards and present their edges to the sun, thus escaping 
injury. On the other hand, the leaflets of Averrhoa and 
Oxalis, when similarly exposed, bend downwards. 

It was shown in the last chapter that heliotropism is 
a modified form of circumnutation ; and as every grow- 
ing part of every plant circumnutates more or less, we 
can understand how it is that the power of bending to 
the light has been acquired by such a multitude of plants 
throughout the vegetable kingdom. The manner in 
which a circumnutating movement — that is, one con- 
sisting of a succession of irregular ellipses or loops — is 
gradually converted into a rectilinear course towards the 
light, has been already explained. First, we have a suc- 
cession of ellipses with their longer axes directed towards 
the light, each of which is described nearer and nearer 
to its source ; then the loops are drawn out into a strong- 
ly pronounced zigzag line, with here and there a small 
loop still formed. At the same time that the movement 
towards the light is increased in extent and accelerated. 



* Wiesner has made remai-ks to tracted from B. Ixxvii (1S7S), 

nearly the same cfTect with respect Sitbz. der k. Akad. der Wisseiisch,, 

to leaves: 'Die nndulirende Nu- Wien. 
tatioii der Internodicn,' p. G, ox- 



496 CONCLUDING REMARKS. Chap. IX. 

that in the opposite direction is lessened and retarded, 
and at last stopped. The zigzag movement to either side 
is likewise gradually lessened, so that finally the course 
becomes rectilinear. Thus under the stimulus of a fairly 
bright light there is no useless expenditure of force. 

As with plants every character is more or less varia- 
ble, there seems to be no great difficulty in believing that 
their circumnutating movements may have been increased 
or modified in any beneficial manner by the preservation 
of varying individuals. The inheritance of habitual 
movements is a necessary contingent for this process of 
selection, or the survival of the fittest ; and we have seen 
good reason to believe that habitual movements are in- 
herited by plants. In the case of twining species the 
circumnutating movements have been increased in am- 
plitude and rendered more circular ; the stimulus being 
here an internal or innate one. With sleeping plants the 
movements have been increased in amplitude and often 
changed in direction ; and here the stimulus is the alter- 
nation of light and darkness, aided, however, by inherit- 
ance. In the case of heliotropism, the stimulus is the 
unequal illumination of the two sides of the plant, and 
this determines, as in the foregoing cases, the modifica- 
tion of the circumnutating movement in such a manner 
that the organ bends to the light. A plant which has 
been rendered heliotropic by the above means, might 
readily lose this tendency, judging from the cases al- 
ready given, as soon as it became useless or injurious. 
A species which has ceased to be heliotropic might also 
be rendered apheliotropic by the preservation of the in- 
dividuals which tended to circumnutate (though the 
cause of this and most other variations is unknow^n) in a 
direction more or less opposed to that whence the light 
proceeded. In like manner a plant might be rendered 
diaheliotropic. 



CHAPTER X. 

Modified Circumnutation : Movements excited by 
Gravitation. 

Means of observation — Apogeotropism — Cjrtisus — Verhena — Beta — 
Gradual conversion of the movement of circumnutation into apo- 
geotropism in Eubus, Lilium, Plialaris, A vena, and Brassica — 
Apogeotropism retarded by heliotropism — Effected by the aid of 
joints or pulvini — Movements of flower-peduncles of Oxalis — Gen- 
eral remarks on apogeotropism — Geotropism — Movements of radi- 
cles — Burying of seed-capsules — Use of process — Trifolium subter- 
raneum — Arachis — Amphicarpsea — Diageotropism — Conclusion. 

Our object in the present chapter is to show that 
geotropism, apogeotropism, and diageotropism are modi- 
fied forms of circumnutation. Extremely fine filaments 
of glass, bearing two minute triangles of paper, were fixed 
to the summits of young stems, frequently to the hypo- 
cotyls of seedlings, to flower-peduncles, radicles, &c., and 
the movements of the parts were then traced in the man- 
ner already described on vertical and horizontal glass- 
plates. It should be remembered that as the stems or 
other parts become more and more oblique with respect 
to the glasses, the figures traced on them necessarily be- 
come more and more magnified. The plants were pro- 
tected from light, excepting whilst each observation was 
being made, and then the^light, which was always a dim 
one, was allowed to enter so as to interfere as little as 
possible with the movement in progress ; and we did not 
detect any evidence of such inteiference. 

When observing the gradations between circumnuta- 
tion and heliotropism, we had the great advantage of 

497 



498 MODIFIED CmCUMNUTATION. Chap. X. 

being able to lessen the light ; but with geotropism anal- 
ogous experiments were of course impossible. . We could, 
however, observe the movements of stems placed at first 
only a little from the perpendicular, in which case ge- 
otropism did not act with nearly so much power, as when 
the stems were horizontal and at right angles to the force. 
Plants, also, were selected which were but feebly geotrop- 
ic or apogeotropic, or had become so from having grown 
rather old. Another plan was to place the stems at first 
so that they pointed 30 or 40 degrees beneath the hori- 
zon, and then apogeotropism had a great amount of work 
to do before the stem was rendered upright ; and in this 
case ordinary circumnutation was often not wholly oblit- 
erated. Another plan was to observe in the evening 
plants which during the day had become greatly curved 
heliotropically ; for their stems under the gradually 
waning light very slowly became upright through the 
action of apogeotropism ; and in this case modified cir- 
cumnutation was sometimes well displayed. 

Apogeotropism. — Plants were selected for observation almost 
by chance, excepting that they were taken from widely different 
famiUes. If the stem of a plant which is even moderately sen- 
sitive to apogeotropism be placed horizontally, the upper grow- 
ing part bends quickly upwards, so as to become perpendicular ; 
and the line traced by joining the dots successively made on a 
glass-plate, is generally almost straight. For instance, a young 
Cytisus fragrans, 13 inches in height, was placed so that the 
stem projected 10° beneath the horizon, and its course was 
traced during 72 h. At first it bent a very little downwards 
(Fig. 182), owing no doubt to the weight of the stem, as this 
occurred with most of the other plants observed, though, as 
they were of course circum nutating, the short downward lines 
were often oblique. After three-quarters of an hour the stem 
began to curve upwards, quickly during the first two hours 
but much more slowly during the afternoon and night, and on 
the following day. During the second night it fell a little, 



Chap. X. 



APOGEOTROPISM. 



499 



and circumnutated during 
the following da}^; but it 
also moved a short distance 
to the right, which was 
caused by a little liglit hav- 
ing been accidentally ad- 
mitted on this side. The 
stem was now inclined 60° 
above the horizon, and had 
therefore risen 70°. With 
time allowed it would prob- 
ably have become upright, 
and no doubt would have 
continued circumnutating. 
The sole remarkable feature 
in the figure here given is 
the straightness of the course 
pursued. The stem, how- 
ever, did not move upwards 
at an equable rate, and it 
sometimes stood almost or 
quite still. Such periods 
probably represent attempts 
to circumnutate in a direc- 
tion opposite to apogeotrop- 
ism. 

The herbaceous stem of 
a Verbena melindres (?) laid 
horizontally, rose in 7 h. 
so much that it could no 
longer be observed on the 
vertical glass which stood 
in front of the plant. 
The long line which was 
traced was almost absolutely 
straight. After the 7 h. it 
still continued to rise, but 
now circumnutated slightly. 
On the following day it 



Fig. 182. 




Cytisus frnqrans : apogootropic move- 
ment of stem from 10° beneath to 
60° above horizon, traced on ver- 
tical glass, from 8.30 a.m. March 
12th to 10.30 P.M. 13tb. The sul)- 
sequcnt circumnntating movement 
is likewise shown up to 6.4.') a.m. 
on the 15th. Nocturnal course rep- 
res(>nted, as usual, by a broken line. 
IMoveiiient not greatly magnified, 
and tracing reduced to two-thirds 
of original scale. 



500 MODIFIED CmCUMNUTATION. Chap. X. 

stood upright, and circumnutated regularly, as shown in Fig. 
82, given in the fourth chapter. The stems of several other 
plants which were highly sensitive to apogeotropisni rose up in 
almost straight lines, and then suddenly began to circumnu- 
tate. A partially etiolated and somewhat old hypocotyl of a 
seedling cabbage (2f inches in height) was so sensitive that 
when placed at an angle of only 23° from the perpendicular, 
it became vertical in 33 minutes. As it could not have been 
strongly acted upon by apogeotropism in the above slightly 
inclined position, we expected that it would have circum- 
nutated, or at least have moved in a zigzag course. Accord- 
ingly, dots were made every 3 minutes; but, vv'hen these were 
joined, the line was nearly straight. After this h\-pocotyl had 
become upright it still moved onwards for half an hour in the 
same general direction, but in a zigzag manner. During the 
succeeding 9 h. it circumnutated regularly, and described 3 
large ellipses. In this case apogeotropism, although acting at 
a very unfavourable angle, quite overcame the ordinary cir- 
cumnutating movement. 

The hypocotyls of Beta vulgaris are highly sensitive to apo- 
geotropism. One was placed so as to project 19° beneath the 
horizon; it fell at first a very little (see Fig. 183), no doubt 
owing to its weight ; but as it was circumnutating the line was 
oblique. During the next 3 h. 8 m. it rose in a nearly straight 
line, passing through an angle of 109°, and then (at 12.3 p.m.) 
stood upright. It continued for 55 m. to move in the same 
general direction beyond the perpendicular, but in a zigzag 
course. It returned also in a zigzag line, and then circumnu- 
tated regularly, describing three large ellipses during the re- 
mainder of the day. It should be observed that the ellipses in 
this figure are exaggerated in size, relatively to the length of 
the upward straight line, owing to the position of the vertical 
and horizontal glass-plates. Another and somewhat old hypo- 
cotyl was placed so as to stand at only 31° from the perpen- 
dicular, in which position apogeotropism acted on it with little 
force, and its course accordingly was slightly zigzag. 

The sheath-like cotyledons of PJialaris Canariensis are 
extremely sensitive to apogeotropism. One was placed so 
as to project 40"* beneath the horizon. Although it was 



Chap. X. 



APOGEOTROPISM. 



501 



rather old and 1'3 inch in 
height, it became verti- 
cal in 4 h. 30 m., having 
passed through an angle 
of 130° in a nearly straight 
line. It then suddenly 
began to circumnutate in 
the ordinary manner. 
The cotyledons of this 
plant, after the first leaf 
has begun to protrude, 
are but slightly apoge- 
otropic, though they still 
continue to circumnutate. 
One at this stage of de- 
velopment was placed 
horizontally, and did not 
become upright even aft- 
er 13 h., and its course 
was slightly zigzag. So, 
again, a rather old hypo- 
cotyl of Cassia t(yra {\\ 
inch in height) required 
28 h. to become upright, 
and its course was dis- 
tinctly zigzag ; whilst 
younger hypocotyls moved 
much more quickly and 
in a nearly straight line. 

When a horizontally 
placed stem or other or- 
gan rises in a zigzag line, 
we may infer from the 
many cases given in our 
previous chapters, that we 
have a modified form of 
circumnutation ; but when 
the course is straight, 
there is no evidence of 



Fig. 183. 



7J2:53'p.m. 



ram.^O''! 



t/i 



Beta vvlqarh : a^pogootropic Tnovomont 
of hypoootyl from 19° bcneatli horizon 
to a vertical position, witli sul)S('(|n('i»t 
circumnutation, traced on a vertical 
and on a horizontal glass-phite. from 
8.28 A.M.Sept. 28th to 8.40 A.M. 2{Mli. 
Figure reduced to one-third of origi- 
nal scale. 



502 



MODIFIED CIRCUMNUTATION. 



Chap. X. 



circumnutation, and any one might maintain that this latter 

movement had been replaced by one of a wholly distinct kind. 

This view seems the more probable when 
Fig 184 

» ■ „ (as sometimes occurred with the hypocotyls 

1 2 of Brassica and Beta, the stems of Cucurbita, 

i, *2 and the cotyledons of Phalaris) the part in 

" ^ question after bending up in a straight course, 

g g suddenly begins to circumnutate to the full 

^.-^ extent and in the usual manner. A fairly 

■^•S good instance of a sudden change of this 

bc'l kind — that is, from a nearly straight upward 

V ^ movement to one of circumnutation — is shown 

'^'M in Fig. 183; but more striking instances were 

Ji5 occasionally observed with Beta, Brassica, and 

r-H '^ Phalaris. 

a CO 

•B o We will now describe a few cases in which 

^-2 it may be seen how gradually circumnutation 

=^1 becomes changed into apogeotropism, under 

o r^ circumstances to be specified in each instance. 

g '^ Bubus idcEus (hybrid). — A young plant, 11 

^ 3 inches in height, growing in a pot, was placed 

g~S; horizontally ; and the upward movement was 

M^ traced during nearly 70 h. ; but the plant, 

°^" though growing vigorously, was not highly 

o g sensitive to apogeotropism, or it was not 

o < capable of quick movement, for during the 

2 o above time it rose only 67°. We may see in 

o-^ the diagram (Fig. 184) that during the first 

g S day of 13 h. it rose in a nearly straight line. 

g =^ When placed horizontally, it was evidently 

o ^ circumnutating, for it rose at first a little, not- 

^ g withstanding the weight of the stem, and then 

'^< sank down; so that it did not start on its 

rS '^ permanently upward course until 1 h. 25 m. 

tS-'^ had elapsed. On the second day, by which 

s o time it had risen considerably, and when apo- 

.'S geotropism acted on it with somewhat less 

J power, its course during 15^ h. was clearly 

f^ zigzag, and the rate of the upward movement 



Chap. X. 



APOGEOTROPISM. 



503 



Fig. 185. 




was not equable. During the third day, also of 15^ h., when 
apogeotropism acted on it with still less power, the stem 
plainly circumnutated, for it moved dur- 
ing this day 3 times up and 3 times 
down, 4 times to the left and 4 to the 
right. But the course was so complex 
that it could hardly be traced on the 
glass. We can, however, see that the suc- 
cessively formed irregular ellipses rose 
higher and higher. Apogeotropism 
continued to act on the fourth morn- 
ing, as the stem was still rising, though 
it now stood only 23° from the perpen- 
dicular. In this diagram the several 
stages may be followed by which an al- 
most rectilinear, upward, apogeotropic 
course first becomes zigzag, and then 
changes into a circumnutating move- 
ment, with most of the successively 
formed, irregular ellipses directed up- 
wards. 

Lilimn auratum. — A plant 23 inches 
in height was placed horizontally, and 
the upper part of the stem rose 58° in 
46 h., in the manner shown in the ac- 
companying diagram (Fig. 185). We 
here see that during the whole of the 
second day of 15^ h., the stem plainly 
circumnutated whilst bending upwards 
through apogeotropism. It had still to 
rise considerably, for when the last dot 
in the figure was made, it stood 32° 
from an upright position. 

PJialaris Canariemis. — A cotyledon 
of this plant (1-3 inch in height) has al- 
ready been described as rising in 4 h. 
30 m. from 40^" beneath the horizon into 
a vertical position, passing tlirough an 
angle of 130° in a nearly straight line, 
33 




Lilmm auratum : apoge- 
otropic niovenicnt of 
stem, traced on a ver- 
tical glass (luring 2 
(lays and 2 nights, 
fn'mi 10.40 A.M. INIarch 
ISth to 8 A.M. 20tli. 
Figure reduced to 
one-half of the origi- 
nal scale. 



504: 



MODIFIED CIRCUMNUTATION. 



Chap. X. 



Fig. 186. 



tri&crmu 



and then abruptly be- 
ginning to circumnutate. 
Another somewhat old 
cotyledon of the same 
height (but from which 
a true leaf had not yet 
protruded), was similar- 
ly placed at 40° beneath 
the horizon. For the 
first 4 h. it rose in a 
nearly straight course 
(Fig. 186), so that by 
1.10 P.M. it was highly 
inclined, and now apo- 
geotropism acted on it 
with much less power 
than before, and it be- 
gan to zigzag. At 4. 15 
P.M. (i.e. in 7 h. from 
the commencement) it 
stood vertically, and aft- 
erwards continued to 
circumnutate in the 
usual manner about the 
same spot. Here then 
we have a graduated 
change from a straight 
upward apogeotropic 
course into circumnuta- 
tion, instead of an abrupt 
change, as in the former 
case. 

Acena sativa. — The 
sheath - like cotyledons, 
whilst young, are strong- 
ly apogeotropic ; and 



Phalaris Canariensis : apogeotropic move , -i 

ment of cotyledon, traced on a vertical some which were placed 
and horizontal glass, from 9.10 a.m. 
Sept. 19tli to 9 A.M. 20th. Figure here 
reduced to one-fifth of original scale. 



at 45° beneath the hori- 
zon rose 90° in 7 or 8 h. 



Chap. X. 



APOGEOTROPISM. 



505 



Fig. 187. 



in lines almost absolutely straight. An oldish cotyledon, from 
which the first leaf began to protrude whilst the following 

observations were being made, 
was placed at 10° beneath the 
horizon, and it rose only 59° in 
24 h. It behaved rather differ- 
ently from any other plant, ob- 
served by us, for during the 
first 4|^ h. it rose in a line not 
far from straight; during the 
next 6^ h. it circumnutated, 
that is, it descended and again 
ascended in a strongly marked 
zigzag course ; it then resumed 
its upward movement in a mod- 
erately straight line, and, with 
time allowed, no doubt would 
have become upright. In this 
case, after the first 4|^ h., ordi- 
nary circumnutation almost 
completely conquered for a 
time apogeotropism. 

Brassica oleracea. — The hy- 
pocotyls of several young seed- 
lings placed horizontally, rose 
up vertically in the course of 6 
or 7 h. in nearly straight lines. 
A seedling which had grown 
in darkness to a height of 2\ 
inches, and w^as therefore rather 
old and not highly sensitive, 
was placed so that the hypo- 
cotyl projected at between 30° 
and 40° beneath the horizon. 
The upper part alone became 
curved upwards, and rose dur- 
ing the first 3 h. 10 m. in a 
nearly straight line (Fig. 187); 
but it was not possible to trace 




Brassica oleracea : apogeotropic 
movement of hypocotyl, traced 
on vertical glass, from 9.20 a.m. 
Sept. 12th to 8.30 A.M. 13th. 
The upx)er part of tlie figure is 
more magnified than the lower 
part. If the whole course had 
been traced, the straight up- 
right line would liav(^ been 
much longer. Figure liere re- 
duced to one-third of the origi- 
nal scale. 



506 MODIFIED CmCUMNUTATION. Chap. X. 

the upward movement on the vertical glass for the first 1 h. 
10 m., so that the nearly straight line in the diagram ought 
to have been much longer. During the next 11 h. the hypo- 
cotyl circumnutated, describing irregular figm-es, each of which 
rose a little above the one previously formed. During the 
night and following early morning it continued to rise in a 
zigzag course, so that apogeotropism was still acting. At the 
close of our observations, after 33 h. (represented by the highest 
dot in the diagram) the hypocotyl was still 32° from the perpen- 
dicular. There can be little doubt that it would ultimately have 
become upright by describing an additional number of irregu- 
lar ellipses, one above the other. 

Apogeotropism retarded hy Heliotropism. — When the stem of 
any plant bends during the day towards a lateral light, the 
movement is opposed by apogeotropism ; but as the light grad- 
ually wanes in the evening the latter power slowly gains the 
upper hand, and draws the stem back into a vertical position. 
Here then we have a good opportunity for observing how apo- 
geotropism acts when very nearly balanced by an opposing 
force. For instance, the plumule of Tropceolum ma^us (see for- 
mer Fig. 175) moved towards the dim evening light in a 
slightly zigzag line until 6.45 p.m., it then returned on its 
coarse until 10.40 p.m., during which time it zigzagged and 
described an ellipse of considerable size. The hypocotyl of 
Brassica oleracea (see former Fig. 173) moved in a straight line 
to the light until 5.15 p.m., and then from the light, making in 
its backward course a great rectangular bend, and then returned 
for a short distance towards the former source of the light; no 
observations were made after 7.10 p.m., but during the night 
it recovered its vertical position. A hypocotyl of Cassia tor a 
moved in the evening in a somewhat zigzag line towards the 
failing light until 6 p.m., and was now bowed 20° from the 
perpendicular; it then returned on its course, making before 
10.30 p.m. four great, nearly rectangular bends and almost com- 
pleting an ellipse. Several other analogous cases were casually 
observed, and in all of them the apogeotropic movement could 
be seen to consist of modified circumnutation. 

Apogeotropic Movements effected 1)1/ tlie aid of joints or pul- 
'cini. — Movements of this kind are well known to occur in the 



Chap. X. APOGEOTROPISM. 5O7 

Graminese, and are effected by means of the thickened bases 
of their sheathing leaves; the stem within being in this part 
thinner than elsewhere.* According to the analogy of all other 
pulvini, such joints ought to continue circumnutating for a 
long period, after the adjoining parts have ceased to grow. AVe 
therefore wished to ascertain whether this was the case with 
the Graminese ; for if so, the upward curvature of their stems, 
when extended horizontally or laid prostrate, would be ex- 
plained in accordance with our -new — namely, that apogeotro- 
pism results from modified circumnutation. After these joints 
have curved upwards, they are fixed in their new position by 
increased growth along their lower sides. 

Loliuni pereiine. — A young stem, 7 inches in height, consist- 
ing of 3 internodes, with the flower-head not yet jDrotruded, 
was selected for observation. A long and very thin glass fihi- 
ment was cemented horizontally to the stem close above the 
second joint, 3 inches above the ground. This joint was subse- 
quently proved to be in an active condition, as its lower side 
swelled much through the action of apogeotropism (in the 
manner described by De Vries) after the haulm had been fas- 
tened down for 24 h. in a horizontal position. The pot was 
so placed that the end of the filament stood beneath the 2-inch 
object-glass of a microscope with an eye-piece micrometer, each 
division of which equalled ^^ of an inch. The end of the fila- 
ment was repeatedly observed during 6 h., and was seen to be 
in constant movement; and it crossed 5 divisions of the microm- 
eter (y^o inch) in 2 h. Occasionally it moved forwards by 
jerks, some of which were yoVo i°ch in length, and then slowly 
retreated a little, afterwards again jerking forwards. These 
oscillations were exactly like those described under Brassica 
and Dionaea, but they occurred only occasionally. We may 
therefore conclude that this moderately old joint was continu- 
ally circumnutating on a small scale. 

'Alopecuruspratensis.—Kjo\xng\Aiix\t, 11 inches in height, with 
the flower-head protruded, but with the florets not yet expanded, 
had a glass filament fixed close above the second joint, at a 



* This structure has boon ro- die, Aufrichtunp: dcs RolaKcrtiTi 
cently dcscrn)cd by Dc Vrios in Gotroidcs,' in ' Landwirthscliaft- 
an interesting article, ' Uebcr licho .Talnl)Uchor,' 1880, p. 473. 



508 



MODIFIED CmCUMNUTATION. 



Chap. X. 



height of only 2 inches above the ground. The basal internode, 
2 inches in length, was cemented to a stick to prevent any 
possibility of its circumnutating. The extremity of the filament, 
which projected about 50° above the horizon, was often observed 
during 24 h. in the same manner as in the last case. Whenever 
looked at, it was always in movement, and it crossed 30 divisions 
of the micrometer {-^\ inch) in 3| h. ; but it sometimes moved 
at a quicker rate, for at one time it crossed 5 divisions in 1^ h. 
The pot had to be moved occa?gionally, as the end of the filament 
travelled beyond the field of vision; but as far as we could 
judge it followed during the daytime a semicircular course; 
and it certainly travelled in two different directions at right 
angles to one another. It sometimes oscillated in the same 
manner as in the last species, some of the jerks forwards being 
as much as -^-^qq of an inch. We may therefore conclude that 
the joints in this and the last species of grass long continue to 
circumnutate ; so that this movement would be ready to be 
converted into an apogeotropic movement, whenever the stem 
was placed in an inclined or horizontal position. 

Movements of the Flower-peduncles of Oxalis carnosa^ due to 
apogeotropism and other forces. — The movements of the main 
peduncle, and of the three or four sub-peduncles which each 
main peduncle of this plant bears, are extremely complex, and 
are determined by several distinct causes. Whilst the flowers 
are expanded, both kinds of peduncles circumnutate about the 
same spot, as we have seen (Fig. 91) in the fourth chapter. 
But soon after the flowers have begun to wither the sub- 
peduncles bend downwards, and this is due to epinasty; for 
on two occasions when pots were laid horizontally, the sub- 
peduncles assumed the same position relatively to the main 
peduncle, as would have been the case if they had remained 
upright; that is, each of them formed with it an angle of 
about 40°. If they had been acted on by geotropism or apheliot- 
ropism (for the plant was illuminated from above), they would 
have directed themselves to the centre of the earth. A main 
peduncle was secured to a stick in an upright position, and one 
of the upright sub-peduncles which had been observed circum- 
nutating whilst the flower was expanded, continued to do so for 
at least 24 h, after it had withered. It then began to bend 



Chap. X. 



APOGEOTEOPISIVL 



509 



downwards, and after 36 h, pointed a little beneath the horizon. 
A new figure was now begun (A, Fig. 188), and the sub-peduncle 



Fig. 188. 





Oxalis carnosa: movements of fl..w(>r peduncle, t>-\*-^'^| .;;;;,,f,,X'iim 
elass A, epinastic downward movement; B, eiu unnmtatiou 
whilst depending vertically ; C, subseciuent upward movement, 
due to apogeotropism and hyponasty combined. 



510 MODIFIED CmCUMNUTATION. Chap. X. 

was traced descending in a zigzag line from 7.20 p.m. on the 19th 
to 9 A.M. on the 22nd. It now pointed almost perpendicularly 
downwards, and the glass filament had to be removed and 
fastened transversely across the base of the young capsule. 
We expected that the sub-peduncle would have been motionless 
in its new position ; but it continued slowly to swing, like a 
pendulum, from side to side, that is, in a plane at right angles 
to that in which it had descended. This circumnutating move- 
ment was observed from 9 a.m. on 22nd to 9 a.m. 24th, as shown 
at B in the diagram. We were not able to observe this par- 
ticular sub-peduncle any longer; but it would certainly have 
gone on circumnutating until the capsule was nearly ripe (which 
requires only a short time), and it would then have moved 
upwards. 

The upward movement (C, Fig. 188) is effected in part by 
the whole sub-peduncle rising in the same manner as it had pre- 
viously descended through epinasty — namely, at the joint where 
united to the main peduncle. As this upward movement 
occurred with plants kept in the dark and in whatever position 
the main peduncle was fastened, it could not have been caused 
by heliotropism, or apogeotropism, but by hyponasty. Besides 
this movement at the joint, there is another of a very different 
kind, for the sub-peduncle becomes upwardly bent in the middle 
part. If the sub-peduncle happens at the time to be inclined 
much downwards, the upward curvature is so great that the 
whole forms a hook. The upper end bearing the capsule, thus 
always places itself upright, and as this occurs in darkness, and 
in whatever position the main peduncle may have been secured, 
the upward curvature cannot be due to heliotropism or hypo- 
nasty, but to apogeotropism. 

In order to trace this upward movement, a filament was 
fixed to a sub-peduncle bearing a capsule nearly ripe, which 
was beginning to bend upwards by the two means just de- 
scribed. Its course was traced (see C, Fig. 188) during 53 h., 
by which time it had become nearly upright. The course is 
seen to be strongly zigzag, together with some little loops. 
We may therefore conclude that the movement consists of 
modified circumnutation. 

The several species of Oxalis probably profit in the following 



Chap. X. APOGEOTROPISM. 511 

manner by their sub-i:)eduncles first bending- downwards and 
then upwards. They are known to scatter their seeds by the 
bursting of the capsule ; the walls of which are so extremely 
thin, like silver paj)er, that they w^ould easily be permeated by 
rain. But as soon as the petals wither, the sepals rise up and 
enclose the young capsule, forming a perfect roof over it as 
soon as the sub-peduncle has bent itself downwards. By its 
subsequent upward movement, the capsule stands when ripe 
at a greater height above the ground by twice the length of the 
sub-peduncle, than it did when dependent, and is thus able 
to scatter its seeds to a greater distance. The sepals, which 
enclose the ovarium whilst it is young, present an additional 
adaptation by expanding widely when the seeds are ripe, so as 
not to interfere with their dispersal. In the case of Oxalis 
acetosella, the capsules are said sometimes to bury themselves 
under loose leaves or moss on the ground, but this cannot occur 
with those of 0. carnosa, as the woody stem is too high. 

Oxalis acetosella. — The peduncles are furnished with a joint in 
the middle, so that the lower part answers to the main peduncle, 



Fig. 189, 




Oxalis acetosella: course pursued by the upper part of a peduncle, 
whilst risino:, traced from 11 a.m. .Tune 1st to 9 a.m. 3rd. Figure 
here reduced to one-half of the original scale. 

and the upper part to one of the sub-peduncles of 0. carnnm. 
The upper part bends downwards, after the flower has begun 
to wither, and the whole peduncle then forma a hook ; that 
this bending is due to epinasty we may infer from the case of 
0. carnosa. When the pod is nearly ripe, the upper part 



512 MODIFIED CmCUMNUTATION. Chap. X. 

straightens itself and becomes erect ; and this is due to hypo- 
nasty or apogeotropism, or both combined, and not to heliot- 
ropism, for it occurred in darkness. The short, hooked part of 
the peduncle of a cleistogamic flower, bearing a pod nearly ripe, 
was observed in the dark during three days. The apex of the 
pod at first pointed perpendicularly down, but in the course of 
three days rose 90°, so that it now projected horizontally. The 
course during the two latter days is shown in Fig. 189; and 
it may be seen how greatly the peduncle, w^hilst rising, circum- 
nutated. The lines of chief movement were at right angles 
to the plane of the originally hooked part. The tracing was 
not continued any longer ; but after two additional days, the 
peduncle with its capsule had become straight and stood 
upright. 



Concluding Remarks on Apogeotropism. — When apo- 
geotropism is rendered by any means feeble, it acts, as 
shown in the several foregoing cases, by increasing the 
always present circumnntating movement in a direction 
opposed to gravity, and by diminishing that in the direc- 
tion of gravity, as well as that to either side. The upward 
movement thus becomes unequal in rate, and is some- 
times interrupted by stationary periods. Whenever 
irregular ellipses or loops are still formed, their longer 
axes are almost always directed in the line of gravit}^, 
in an analogous manner as occurred with heliotropic 
movements in reference to the light. As apogeotropism 
acts more and more energetically, ellipses or loops cease 
to be formed, and the course becomes at first strongly, 
and then less and less zigzag, and finally rectilinear. 
From this gradation in the nature of the movement, and 
more especially from all growing parts, which alone (ex- 
cept when pulvini are present) are acted on by apoge- 
otropism, continually circumnntating, we may conclude 
that even a rectilinear course is merely an extremely 
modified form of circumnutation. It is remarkable that 



Chap. X. APOGEOTROPISM. 5I3 

a stem or other organ which is highly sensitive to apo- 
geotropism, and which has bowed itself rapidly upwards 
in a straight line, is often carried beyond the vertical, 
as if by momentum. It then bends a little backwards 
to a point round which it finally circumnutates. Two 
instances of this were observed with the hypocotyls of 
Beta vulgaris^ one of which is shown in Fig. 183, and 
two other instances with the hypocotyls of Brassica. 
This momentum-like movement probably results from 
the accumulated effects of apogeotropism. For the 
sake of observing how long such after-effects lasted, a 
pot with seedlings of Beta was laid on its side in the 
dark, and the hypocotyls in 3 h. 15 m. became highly 
inclined. The pot, still in the dark, was then placed 
upright, and the movements of the two hypocotyls were 
traced ; one continued to bend in its former direction, 
now in opposition to apogeotropism, for about 37 m., 
perhaps for 48 m. ; but after 61 m. it moved in an 
opposite direction. The other hypocotyl continued to 
move in its former course, after being placed upright, 
for at least 37 m. 

Different species and different parts of the same 
species are acted on by apogeotropism in very different 
degrees. Young seedlings, most of which circum nu- 
tate quickly and largely, bend upwards and become 
vertical in much less time than do any older plants 
observed by us ; but whether this is due to their 
greater sensitiveness to apogeotropism, or merely to 
their greater flexibility we do not know. A hypocotyl 
of Beta traversed an angle of 109° in 3 h. 8 m., and a 
cotyledon of Phalaris an angle of 130° in 4 h. 30 m. On 
the other hand, the stem of a herbaceous Verbena rose 
90° in about 24 h. ; that of Rubus G7°, in 70 h. ; that of 
Cytisus 70°, in 72 h. ; that of a young American Oak 
only 37°, in 72 h. The stem of a young Cijperus alter- 



514 



MODIFIED CIRCUMNUTATION. 



Chap. X. 



nifoUus rose only 11° in 96 h. ; the bending being 
confined to near its base. Though the sheath-like coty- 
ledons of Phalaris are so extremely sensitive to apoge- 
otropism, the first true leaves which protrude from them 
exhibited only a trace of this action. Two fronds of a 
fern, Nephrodium 7nolle, both of them young and one 
with the tip still inwardly curled, were kept in a hori- 
zontal position for 46 h., and during this time they rose 
so little that it was doubtful whether there was any true 
apogeotropic movement. 

The most curious case known to us of a difference 
in sensitiveness to gravitation, and consequently of 
movement, in different parts of the same organ, is that 
offered by the petioles of the cotyledons of Ipomoea lep- 
tophylla. The basal part for a short length where 
united to the undeveloped hypocotyl and radicle is 
strongly geotropic, whilst the whole upper part is 
strongly apogeotropic. But a portion near the blades 
of the cotyledons is after a time acted on by epinasty 
and curves downwards, for the sake of emerging in the 
form of an arch from the ground ; it subsequently 
straightens itself, and is then again acted on by apo- 
geotropism. 

A branch of Cucurhita ovifera^ placed horizontally, 
moved upwards during 7 h. in a straight line, until it 
stood at 40° above the horizon ; it then began to cir- 
cumnutate, as if owing to its trailing nature it had no 
tendency to rise any higher. Another upright branch 
was secured to a stick, close to the base of a tendril, 
and the pot was then laid horizontally in the dark. 
In this position the tendril circumnutated and made 
several large ellipses during 14 h., as it likewise did 
on the following day; but during this whole time it 
was not in the least affected by apogeotropism. On the 
other hand, when branches of another Cucurbitaceous 



Chap. X. APOGEOTROPISM. 515 

plant, Echinocytis lobata^ were fixed in the dark so that 
the tendrils depended beneath the horizon, these began 
immediately to bend upwards, and whilst thus moving 
they ceased to circumnutate in any plain manner ; but 
as soon as they had become horizontal they recom- 
menced to revolve conspicuonsly.* The tendrils of 
Passifiora gracilis are likewise apogeotropic. Two 
branches were tied down so that their tendrils pointed 
many degrees beneath the horizon. One was observed 
for 8 h., during which time it rose, describing two 
circles, one above the other. The other tendril rose in 
a moderately straight line during the first 4 h., making 
however one small loop in its course ; it then stood at 
about 45° above the horizon, where it circumnutated 
during the remaining 8 h. of observation. 

A part or organ which whilst young is extremely 
sensitive to apogeotropism ceases to be so as it grows 
old ; and it is remarkable, as showing the independence 
of this sensitiveness and of the circumnutating move- 
ment, that the latter sometimes continues for a time 
after all power of bending from the centre of the earth 
has been lost. Thus a seedling Orange bearing only 3 
young leaves, with a rather stiff stem, did not curve 
in the least upwards during 24 h. whilst extended hori- 
zontally ; yet it circumnutated all the time over a small 
space. The hypocotyl of a young seedling of Cassia 
tora, similarly placed, became vertical in 12 h. ; that 
of an older seedling, 1|- inch in height, became so in 
28 h. ; and that of another still older one, 1^- inch in 
height, remained horizontal daring two days, but dis- 
tinctly circumnutated daring this whole time. 

When the cotyledons of Phalaris or Avena arc laid 



«- For details see 'The Movements aud Habits of Cliuil)iug Plants,' 
1875, p. 131. 



516 MODIFIED CIRCUMNUTATION. Chap. X. 

horizontally, the uppermost part first hends upwards, 
and then the lower part ; consequently, after the lower 
part has become much curved upwards, the upper part 
is compelled to curve backwards in an opposite direc- 
tion, in order to straighten itself and to stand ver- 
tically ; and this subsequent straightening process is 
likewise due to apogeotropism. The upper part of 8 
young cotyledons of Phalaris were made rigid by beiug 
cemented to thin glass rods, so that this part could not 
bend in the least ; nevertheless, the basal part was not 
prevented from curving upward. A stem or other organ 
which bends upwards through apogeotropism exerts 
considerable force ; its own weight, which has of course 
to be lifted, was sufficient in almost every instance to 
cause the part at first to bend a little downwards ; but 
the downward course was often rendered oblique by the 
simultaneous circumnutating movement. The cotyle- 
dons of Avena placed horizontally, besides lifting their 
own weight, were able to furrow the soft sand above 
them, so as to leave little crescentic open spaces on the 
lower sides of their bases ; and this is a remarkable 
proof of the force exerted. 

As the tips of the cotyledons of Phalaris and Avena 
bend upwards through the action of apogeotropism be- 
fore the basal part, and as these same tips when excited 
by a lateral light transmit some influence to the lower 
part, causing it to bend, we thought that the same rule 
might hold good with apogeotropism. Consequently, 
the tips of 7 cotyledons of Phalaris were cut off for a 
length in three cases of -2 inch and in the four other 
cases of -14, -12, -1, and -07 inch. But these cotyledons, 
after being extended horizontally, bowed themselves up- 
wards as effectually as the unmutilated specimens in the 
same pots, showing that sensitiveness to gravitation is 
not confined to their tips. 



Chap. X. GEOTROPISM. 51Y 

Geotropism. 

This movement is directly the reverse of apogeotro- 
pism. Many organs bend downwards through epinasty 
or apheliotropism or from their own weight; but we 
have met with very few cases of a downward movement 
in sub-aerial organs due to geotropism. We shall, how- 
ever, give one good instance in the following section, in 
the case of Trifolium suhterraneum^ and probably in 
that of Arachis hy])ogcBa. 

On the other hand, all roots which penetrate the 
ground (including the modified root-like petioles of Me- 
garrhiza and Ii^onima leijtojpliyllci) are guided in their 
downward course by geotropism ; and so are many aerial 
roots, whilst others, as those of the Ivy, appear to be 
inditferent to its action. In our first chapter the move- 
ments of the radicles of several seedlings were described. 
We may there see (Fig. 1) how a radicle of the cabbage, 
when pointing vertically upwards so as to be very little 
acted on by geotropism, circumnutated ; and how an- 
other (Fig. 2) which was at first placed in an inclined 
position bowed itself downwards in a zigzag line, some- 
times remaining stationary for a time. ""Tiwo other radi- 
cles of the cabbage travelled downwards in almost recti- 
linear courses. A radicle of the bean placed upright 
(Fig. 20) made a great sweep and zigzagged ; but as it 
sank downwards and was more strongly acted on by 
geotropism, it moved in an almost straight course. A 
radicle of Cucurbita, directed upwards (Fig. 20), also 
zigzagged at first, and described small loops ; it tlien 
moved in a straight line. Nearly the same result was 
observed with the radicles of Zea mays. But tlie best 
evidence of the intimate connection between circumnu- 
tation and geotropism was afforded by the radicles of 
Phaseolus, Vicia, and Quercus, and in a less degree by 



518 MODIFIED CIRCUMNUTATION. Chap. X. 

those of Zea and ^sculus (see Figs. 18, 19, 21, 41, and 
52) ; for when these were compelled to grow and slide 
down highly inclined surfaces of smoked glass, they left 
distinctly serpentine tracks. 

The Burying of Seed-capsules : Trifolium subterraneum. — The 
flower-heads of this plant are remarkable for producing only 
3 or 4 perfect flowers, which are situated exteriorly. All the 
other many flowers abort, and are modified into rigid points, 
with a bundle of vessels running up their centres. After a time 
5 long, elastic, claw-like projections, which represent the divi- 
sions of the calyx, are developed on their summits. As soon as 
the perfect flowers wither they bend downwards, supposing the 
peduncle to stand upright, and they then closely surround its 
upper part. This movement is due to epinasty, as is likewise 
the case with the flowers of 7\ repens. The imperfect central 
flowers ultimately follow, one after the other, the same course. 
Whilst the perfect flowers are thus bending down, the whole 
peduncle curves dow^nwards and increases much in length, 
until the flower-head reaches the ground, Vaucher* says that 
when the plant is so placed that the heads cannot soon reach 
the ground, the peduncles grow to the extraordinary length of 
from 6 to 9 inches. In whatever position the branches may be 
placed, the upper part of the peduncle at first bends vertically 
upwards through heliotropism ; but as soon as the flowers 
begin to wither the downward curvature of the whole peduncle 
commences. As this latter movement occurred in complete 
darkness, and with peduncles arising from upright and from 
dependent branches, it cannot be due to apheliotropism or to 
epinasty, but must be attributed to geotropism. Nineteen 
upright flower-heads, arising from branches in all sorts of posi- 
tions, on plants growing in a warm greenhouse, were marked 
wath thread, and after 24 h. six of them w^ere vertically depend- 
ent; these therefore had travelled through 180° in this time. 
Ten were extended sub-horizontally, and these had moved 
through about 90^. Three very young peduncles had as yet 



Hist. Phys. des Plantes d' Europe,' torn. ii. 1841, p. 106. 



Chap X. GEOTEOPISM. 5 19 

moved only a little downwards, but after an additional 24 h. 
were greatly inclined. 

At the time when the flower-heads reach the ground, the 
younger imperfect flowers in the centre are still pressed closely 
together, and forma conical projection ; whereas the perfect and 
imperfect flowers on the outside are upturned and closely sur- 
round the peduncle. They are thus adapted to offer as little 
resistance, as the case admits of, in penetrating the ground, 
though the diameter of the flower-head is still considerable. 
The means by which this penetration is efTected will presently 
be described. The flower heads are able to bury themselves in 
common garden mould, and easily in sand or in fine sifted 
cinders packed i-ather closely. The depth of which they pene- 
trated, measured from the surface to the base of the head, v.as 
between ^ and ^ inch, but in one case rather above OG inch. 
With a plant kept in the house, a head partly buried itself in 
sand in 6 h. : after 3 days only the tips of the reflexed calyces 
were visible, and after 6 days the whole had disappeared. But 
with plants growing out of doors we believe, from casual obser- 
vations, that they bury themselves in a much shorter time. 

After the heads have buried themselves, the central aborted 
flowers increase considerably in length and rigidity, and 
become bleached. They gradually curve, one after the other, 
upwards or towards the peduncle, in the same manner as 
did the perfect flowers at first. In thus moving, the long claws 
on their summits carry with them some earth. Hence a flower- 
head which has been buried for a sufficient time, forms a rather 
large ball, consisting of the aborted flowers, separated from one 
another by earth, and surrounding the little pods (the product 
of the perfect flowers) wiiich lie close round the upjier ytuvt of 
the peduncle. The calyces of the perfect and imperfect flowers 
are clothed with simple and multicellular hairs, which have tlie 
power of absorption; for when placed in a weak sohitioii of 
carbonate of ammonia (2 gr. to 1 oz. of water) their protoj)l:is- 
mic contents immediately became aggregated and afterwards 
displayed the usual slow movements. Tliis clover generally 
grows in dry soil, but whether the power of absorption by the 
hairs on the buried flower-heads is of any imjiortance to them 
we do not know. Only a few of tlie flower-heads, which from 



520 



MODIFIED CIRCUMNUTATION. 



Chap. X. 



their position are not able to reach the ground and bury them- 
selves, yield seeds ; whereas the buried ones never failed, as far 
as we observed, to produce as many seeds as there had been 
perfect flowers. 

We will now consider the movements of the peduncle whilst 
curving down to the ground. We have seen in Chap. IV., 



riff. 190. 



Hani.SS'^^ 




-.^"crjTKBB^S 



ojithX 



TrifoUum suMerraneum : downward movement of peduncle from 19° 'be- 
neath the horizon to a nearly vertically dependent position, traced 
from 11 A.M. July 22nd to the morning of 25tli. Glass filament 
fixed transversely across peduncle, at base of flower-head. 



Fig. 92, p. 225, that an upright young flower-head circumnu- 
tated conspicuously; and that this movement continued after 
the peduncle had begun to bend downwards. The same 
peduncle was observed when inclined at an angle of 19° above 
the horizon, and it circnmnutated during two days. Another 
which was already curved 36° beneath the horizon, was observed 



Chap. X. 



GEOTROPISM. 



521 



Fis 




Trifolium subterraneum : cir- 
cumnutating movement of 
peduncle, whilst the flower- 
head was huryin^ itself in 
sand, with the reflexcd tips 
of the calyx still visible ; 
traced from 8 a.m. July 
26th to 9 A.M. on 27th. 
Glass filament fixed trans- 
versely across peduncle, 
near flower-head. 



from 11 A.-M. July 22nd to the 27th, by which latter date it 
had become vertically dependent. Its course during the first 
12 h. is showm in Fig. 190, and its position on the three suc- 
ceeding mornings until the 2oth, 
when it w^as nearly vertical. Dur- 
ing the first day the peduncle clear- 
ly circumnutated, for it moved 4 
times down and 3 times up; and 
on each succeeding day, as it sank 
downwards, the same movement 
continued, but was only occasion- 
ally observed and was less strongly 
marked. It should be stated that 
these peduncles were observed un- 
der a double skylight in the house, 
and that they generally moved 
downwards very much more slow- 
ly than those on plants growing 
out of doors or in the greenhouse. 
The movement of another ver- 
tically dependent peduncle with the flower-head standing half 
an inch above the ground, was traced, and again when it first 
touched the ground; in both cases irregular ellipses were de- 
scribed every 4 or 5 h. A peduncle on a plant which had been 
brought into the house, moved from an upright into a verti- 
cally dependent position in a single day; and here the cour.se 

during the first 12 h. was nearly 
straight, but with a few well- 
marked zigzags which betrayed 
the essential nature of the move- 
ment. Lastly, the circumnuta- 
tion of a peduncle was traced 
during 51 h. whilst in the act of 
burying itself obliquely in a lit- 
tle heap of sand. After it liad 
buried itself to such a depth that 
the tips of the sepals were alone visible, the above figure (Fig. 
191) was traced during 25 h. When the flower-head liad 
completely disappeared beneath the sand, another tracing was 



Fig. 192. 




Trifolium siihterraneum : move- 
ment of same peduncle, with 
flower-head completely buried 
heneath the sand : traced from 
8 A.M. to 7.15 P.M. on July 29th. 



522 MODIFIED CmCUMNUTATION. Chap. X. 

made during 11 h. 45 m. (Fig. 192); and here again we see that 
the peduncle was circumnutating. 

Any one who will observe a flower-head burying itself, will 
be convinced that the rocking movement, due to the continued 
circum nutation of the peduncle, plays an important part in the 
act. Considering that the flower-heads are very light, that the 
peduncles are long, thin, and flexible, and that they arise from 
flexible branches, it is incredible that an object as blunt as one 
of these flower-heads could penetrate the ground by means of 
the growing force of the peduncle, unless it were aided by the 
rocking movement. After a flower-head has penetrated the 
ground to a small depth, another and efiicieut agency comes into 
play ; the central rigid aborted flowers, each terminating in five 
long claws, curve up toward the peduncle; and in doing so 
can hardly fail to drag the head down to a greater depth, aided 
as this action is by the circumnutating movement, which con- 
tinues after the flower-head has completely buried itself. The 
aborted flowers thus act something like the hands of the mole, 
which force the earth backwards and the body forwards. 

It is well known that the seed-capsules of various widely 
distinct plants either bury themselves in the ground, or are pro- 
duced from imperfect flowers developed beneath the surface. 
Besides the present case, two other well marked instances will 
be immediately given. It is probable that one chief good thus 
gained is the protection of the seeds from animals which prey on 
them. In the case of T. subterraneum^ the seeds are not only 
concealed by being buried, but are likewise protected by being 
closely surrounded by the rigid, aborted flowers. We may the 
more confidently infer that protection is here aimed at, because 
the seeds of several species in this same genus are protected in 
other ways;* namely, by the swelling and closure of the calyx, 
or by the persistence and bending down of the standard- petal, 
&c. But the most curious instance is that of T. globosum, in 
which the upper flowers are sterile, as in T. svHerraneum^ but 
are here developed into large brushes of hairs which envelop 
and protect the seed-bearing flowers. Nevertheless, in all these 
cases the capsules, with their seeds, may profit, as Mr. T. This- 



* Vaucher, 'Hist Phys. des Plantes d'Europe,' torn. ii. p. 110. 



Chap. X. GEOTROPISM. 503 

elton Dyer has remarked,* by their being kept somewhat damp 
and the advantage of such dampness perhaps throws light on 
the presence of the absorbent hairs on the buried Hower-heads 
of T. subterraneum. According to Mr. Bentham, as quoted by 
Mr. Dyer, the prostrate habit of Heliimthemuin prostratum 
" brings the capsules in contact with the surface of the ground, 
postpones their maturity, and so favours the seeds attaining 
a larger size." The capsules of Cyclamen and of Oxalis (iceto- 
sella are only occasionally buried, and tiiis only beneath dead 
leaves or moss. If it be an advantage to a plant that its cap- 
sules should be kept damp and cool by being laid on the ground, 
we have in these latter cases the first step, from which the 
power of penetrating the ground, with the aid of the always 
present movement of circumnutation, might afterwards have 
been gained. 

Arachis hypogcea. — The flowers which bury themselves, rise 
from stiff branches a few inches above the ground, and stand 
upright. After they have fallen off, the gynophore, that is 
the part which supports the ovarium, grows to a great length, 
even to 3 or 4 inches, and bends perpendicularly downwards. 
It resembles closely a peduncle, but has a smooth and ])ointed 
apex, which contains the ovules, and is at first not in the 
least enlarged. The apex after reaching the ground penetrates 
it, in one case observed by us to a depth of 1 inch, and in 
another to 7 inch. It there becomes developed into a large 
pod. Flowers which are seated too high on the plant for tlie 
gynophore to reach the ground are saidt never to produce 
pods. 

The movement of a young gynophore, rather under ?.n inch 
in length and vertically dependent, was traced during 4G h. by 
means of a glass filament (with sights) fixed transversely a 
little above the apex. It plainly circumnutated (Tig. 193) 
whilst increasing in length and growing downwards. It was 
then raised up, so as to be extended almost horizontally, and 
the terminal part curved itself downwards, following a nearly 
straight course during 12 h., but witii one attcmjit to circuin- 



* Sec his intorostinsr nrticlf^ in t'Gard. Chroniek',* 1S57, page 
Nature,' April 4tli, 1878, p. 44G. 5GG. 



624 



MODIFIED CIRCUMNUTATION. 



Chap. X. 



Fig. 193. 




nutate as f-hown in Fig. 194. After 34 h. it had become nearly 
vertical. Whether the exciting cause of the downward move- 
ment is geotropism or apheliotropism was not ascertained; but 
probably it is not apheliotropism, as all the gynophores grew 
straight down towards the ground, whilst the light in the hot- 
house entered from one side as well as from above. Another 
and older gynophore, the apex of which 
had nearly reached the ground, was ob- 
^'s^.,^^ served during 3 days in the same manner 

'^^ ^^ as the first-mentioned short one; and it 

was found to be always circumnutating. 
During the first 34 h. it described a fig- 
ure which represented four ellipses. Last- 
ly, a long gynophore, the apex of which 
had buried itself to the depth of about 
half an inch, was pulled up and extended 
horizontally : it quickly began to curve 
downwards in a zigzag line; but on the 
following day the terminal bleached 
po;gtion was a little shrivelled. As the 
gynophores are rigid and arise from stiS. 
branches, and as they terminate in sharp 
smooth points, it is probable that they 
could penetrate the ground by the mere 
force of growth. But this action must be 
aided by the circumnutating movement, 
for fine sand, kept moist, was pressed close 
round the apex of a gynophore which had 
reached the ground, and after a few hours 
it was surrounded by a narrow open crack. 
After three weeks this gynophore was un- 
covered, and the apex was found at a 
depth of rather above half an inch devel- 
oped into a small, white, oval pod. 

AmpTiicarpcEa monoica. — This plant pro- 
duces long thin shoots, which twine round 
a support and of course circumnutate. Early in the summer 
shorter shoots are produced from the lower parts of the plant, 
which grow perpendicularly downwards and penetrate the 



/ 



/ 



Arachis hypoqppa : cir- 
cumiiutation of ver- 
tically dependent 
young gynophore, 
traeed on a vertical 
glass from 10 a.m. 
July 31st to 8 A.M. 
Aug. 2nd. 



Chap. X. 



DIAGEOTROPISM. 



525 



ground. One of these, terminating in a minute bud, was ob- 
served to bury itself in sand to a depth of 2 inch in 24 h. 
It was lifted up and fixed in an in- 
clined position about 25° beneath the ^^S- 194. 
horizon, being feebly illuminated from ^ 
above. In this position it described two \ 
vertical ellipses in 24 h. ; but on the fol- 
lowing day, when brought into the 
house, it circumnutated only a very lit- 
tle round the same spot. Other branches 
were seen to penetrate the ground, and 
were afterwards found running like 
roots beneath the surface for a length 
of nearly two inches, and they had 
grown thick. One of these, after thus 
running, had emerged into the air. 
How far circumnutation aids these deli- 
cate branches in entering the ground 
we do not know ; but the reflexed hairs 
with which they are clothed will assist 
in the work. This plant produces 
pods in the air, and others beneath the 
ground ; which differ greatly in appear- 
ance. Asa Gray says * that it is the im- 
perfect flowers on the creeping branches 
near the base of the plant which pro- 
duce the subterranean pods; these 
flowers, therefore, must bury themselves 
like those of Arachis. But it may be 
suspected that the branches which were 
seen by us to penetrate the ground also 
produce subterranean flowers and pods. 



DiAGEOTROPISM. 

Besides geotropism and apoge- 
otropism, there is, according to 
Frank, an allied form of movement, 



Arnchis hypoqsea : down- 
ward mov<>m('nt of sanio 
youn<!; pyiiopliorc. aftor 
bcinp ('xtcndfd horizon- 
tally ; traced on a vorti- 
cal {ilass from H.30 A.M. 
to 8.30 P.M. AuR. 2nd. 



'Manual of the Botauyof the Northern United Stati's,' IHTMi, p. lOU. 



526 MODIFIED CmCUMNUTATION. Chap. X. 

namely, " transverse-geotropism," or diageotropism, as 
we may call it for the sake of matching our other terms. 
Under the influence of gravitation certain parts are ex- 
cited to place themselves more or less transversely to the 
line of its action.* We made no observations on this 
subject, and will liere only remark that the position of the 
secondary radicles of various plants, which extend horizon- 
tally or are a little inclined downwards, would probably 
be considered by Frank as due to transverse-geotropism. 
As it has been shown in Chap. I. that the secondary 
radicles of Cucurbita made serpentine tracks on a smoked 
glass-plate, they clearly circumnutated, and there can 
hardly be a doubt tliat this holds good with other sec- 
ondary radicles. It seems therefore highly probable that 
they place themselves in their diageo tropic position by 
means of modified circumnutation. 

Finally, we may conclude that the three kinds of 
movement which have now been described and which 
are excited by gravitation, consist of modified circumnu- 
tation. Different parts or organs on the same plant, and 
the same part in different species, are thus excited to act 
in a widely different manner. We can see no reason 
why the attraction of gravity should directly modify the 
state of turgescence and subsequent growth of one part 
on the upper side and of another part on the lower side. 
We are therefore led to infer that both geotropic, apo- 
geotropic, and diageotropic movements, the purpose of 
which we can generally understand, have been acquired 
for the advantage of the plant by the modification of the 
ever-present movement of circumnutation. This, how- 
ever, implies that gravitation produces some effect on the 
young tissues sufficient to serve as a guide to the plant. 



••■ Elfving has ktely described excellent instance of siicli move- 
CArbeiten des Bot. Instituts in ments in the rhizomes of certain 
Wiirzburg,' B. ii. 1880, p. 489) an plants. 



CHAPTER XL 

Localised Sensitiveness to Gravitation, and its 
Transmitted Effects. 

General considerations — Vicia faba, effects of amputating the tips of 
the radicles — Eegeneration of the tips — Effects of a short exposure 
of the tips to geotropic action and their subsequent amputation — 
Effects of amputating the tips ©bliquely — Effects of cauterising the 
tips — Effects of grease on the tips — Pisum sativum, tips of radicles 
cauterised transversely, and on their upper and lower sides — 
Phaseolus, cauterisation and grease on the tips — Gossypium — 
Cucurbita, tips cauterised transversely, and on their upper and 
lower sides — Zea, tips cauterised— Concluding remarks and sum- 
mary of chapter — Advantages of the sensibility to geotropism being 
localised in the tips of the radicles. 

CiESiELSKi states* that when the roots of Pisnm, 
Lens and Vicia were extended horizontally with their 
tips cut off, they were not acted on by geotropism ; but 
some days afterwards, when a new root-cap and vegeta- 
tive point had been formed, they bent themselves per- 
pendicularly downwards. He further states that if the 
tips are cut off, after the roots have been left extended 
horizontally for some little time, but before they have 
begun to bend downwards, they may be placed in any 
position, and yet will bend as if still acted on by geot- 
ropism ; and this shows that some influence had been 
already transmitted to the bending part from the tip 
before it was amputated. Sachs repeated these experi- 
ments; he cut off a length of between '05 and 1 mm. 
(measured from the apex of the vegetative point) of the 



* 'AbwartskrummuugderWurzcl,' luaug. Dissert., Hrcslau.. 1S71, p. -'!>. 

527 



528 SENSITIVENESS TO GRAVITATION. Chap. XI. 

tips of the radicles of the bean ( Viciafaba), and placed 
them horizontally or vertically in damp air, earth, and 
water, with the result that they became bowed in ail 
sorts of directions.* He therefore disbelieved in Oiesiel- 
ski's conclusions. But as we have seen with several 
plants that the tip of the radicle is sensitive to contact 
and to other irritants, and that it transmits some influ- 
ence to the upper growing part causing it to bend, there 
seemed to us to be no a priori improbability in Ciesiel- 
ski's statements. We therefore determined to repeat his 
experiments, and to try others on several species by dif- 
ferent methods. 

Vicia faba. — Radicles of this plant were extended horizon- 
tally either over water or with their lower surfaces just touching 
it. Their tips had previously been cut off, in a direction as 
accurately transverse as could be done, to different lengths, 
measured from the apex of the root-cap, and which will be 
specified in each case. Light was always excluded. We had 
previously tried hundreds of unrautilated radicles under similar 
circumstances, and found that every one that was healthy be- 
came plainly geotropic in under 13 h. In the case of four radi- 
cles which had their tips cut off for a length of 1-5 mm., new 
root-caps and new vegetative points were re-formed after an 
interval of 3 days 20 h. ; and these when placed horizontally 
were acted on by geotropism. On some other occasions this 
regeneration of the tips and reacquired sensitiveness occurred 
within a somewhat shorter time. Therefore, radicles having 
their tips amputated should be observed in from 12 to 48 h. 
after the operation. 

Four radicles were extended horizontally with their lower 
surfaces touching the water, and with their tips cut off for a 
length of only O'S mm.: after 23 h. three of them were still 
horizontal; after 47 h. one of the three became fairly geotropic; 
and after 70 h. the other two showed a trace of this action. The 
fourth radicle was vertically geotropic after 23 h. ; but by an 



' Arbeiten des Bot. Instituts in Wiirzburg,' Heft iii. 1873, p. 432. 



Chap. XL TRANSMITTED EFFECTS : VICIA. 



529 



accident the root-cap alone and not the vegetative point was 
found to have been amputated; so that this case formed no 
real exception and might have been excli"^^^^'^- 

Five radicles were extended horizon^-^^ly lil^e the last, and 
had their tips cut off for a length of 1 mn^- 5 ^^^er 22-33 h., four 
of them were still horizontal, and one V^^ slightl}^ geotropic; 
after 48 h. the latter had become vertic'^l ? /^ second was also 
somewhat geotropic; two remained app:^""^^"^'^^^^ horizontal; 
and the last or fifth had grown in a disc^^dered manner, for it 
was inclined upwards at an angle of 65" .^oove the horizon. 

Fourteen radicles were extended horizf^"^^^^^' ^^ a little height 
over the water with their tips cut off foi ^ length of 1-5 mm. ; 
after 12 h. all were horizontal, whilst fiV^ control or standard 
specimens in the same jar were all bei^^ greatly downwards. 
After 24 h. several of the amputated r^^^licles remained hori- 
zontal, but some showed a trace of gePtropism, and one was 
plainly geotropic, for it was inclined at 4p° beneath the horizon. 

Seven horizontally extended radicles pom which the tips had 
been cut off for the unusual length of 2 i^^^"^- unfortunately were 
not looked at until 35 h. had elapsed; th^ee were still horizon- 
tal, but, to our surprise, four were more of less plainly geotropic. 

The radicles in the foregoing cases ^^e^e measured before 
their tips were amputated, and in the cd^^'^^e of 24 h. they had 
all increased greatly in length ; but the measurements are not 
worth giving. It is of more importance- ^^^^^ Sachs found that 
the rate of growth of the different parts of radicles with ampu- 
tated tips was the same as with unmutih^tod ones. Altogether 
twenty-nine radicles were operated on ^" ^'^^ manner above 
described, and of these only a few shov^'^'f^ =^"y geotropic cur- 
vature within 24 h. ; whereas radicles ^^'i^h unmutilated tips 
always became, as already stated, much 1^^"^ ^^wn in less than 
half of this time. The part of the radiclr ^^l^if''' '^^'^^^^ "^^st lies 
at the distance of from 3 to 6 mm. fr"'" tlie tip, and as the 
bending part continues to grow after th^ operation, there does 
not seem any reason why it should not l'«'^^'^ '>^'t^" "^'^''^ "" ^^J' 
geotropism, unless its curvature depen(^«'^l "" ^""'<' "I'li'eiK'C 
transmitted from the tip. And we have '^^^'-'^ evidence ai such 
transmission in Ciesielski's experiments, ^^''if^l^ ^^'^ repeated and 
extended in the following manner. 



530 SENSITIVENESS TO GRAVITATION. Chap. XI. 

Beans were embedded in friable peat with the hilum down- 
wards, and after their radicles had grown perpendicularly down 
for a length of from -g- to 1 inch, sixteen were selected which 
were perfectly straight, and these were placed horizontally on 
the peat, being covered by a thin layer of it. They were thus 
left for an average period of 1 h. 37 m. The tips were then 
cut off transversely ioT a length of 1-5 mm., and immediately 
afterwards they were embedded vertically in the peat. In this 
position geotropism would not tend to induce any curvature, but 
if some influence had already been transmitted from the tip to 
the part which bends most, we might expect that this part 
would become curved in the direction in which geotropism had 
previously acted ; for it should be noted that these radicles being 
now destitute of their sensitive tips, would not be prevented by 
geotropism Irom curving in any direction. The result was that 
of the sixteen vertically embedded radicles, four continued for 
several days to grow straight downwards, whilst twelve became 
more or less bowed laterally. In two of the twelve, a trace of 
curvature was perceptible in 3 h. 30 m., counting from the time 
when they had first been laid horizontally ; and all twelve were 
plainly bowed in 6 h., and still more plainly in 9 h. In every 
one of them the curvature was directed towards the side which 
had been downwards whilst the radicles remained horizontal. 
The curvature extended for a length of from 5 to, in one in- 
stance, 8 mm., measured from the cut-off end. Of the twelve 
bowed radicles five became permanently bent into a right angle ; 
the other seven were at first much less bent, and their curvature 
generally decreased after 24 h., but did not wholly disappear. 
This decrease of curvature would naturally follow, if an ex- 
posure of only 1 h. 37 m. to geotropism, served to modify the 
turgescence of the cells, but not their subsequent growth to 
the full extent. The five radicles which were rectangularly 
bent became fixed in this position, and they continued to grow 
out horizontally in the peat for a length of about 1 inch during 
from 4 to 6 days. By this time new tips had been formed ; and 
it should be remarked that this regeneration occurred slower in 
the peat than in water, owing perhaps to the radicles being 
often looked at and thus disturbed. After the tips had been 
regenerated, geotropism was able to act on them, so that they 



Chap. XL TRANS3IITTED EFFECTS : VICIA. 



531 



Fij?. 195. 



now became bowed vertically downwards. An accurate draw- 
ing (Fig. 195) is given below of one of these live radicles, 
reduced to half the natural size. 

We next tried whether a shorter exposure to geotropism 
would suffice to produce an after-effect. Seven radicles were 
extended horizontally for an hour, instead of 1 h. 37 m. as in the 
former trial; and after their 
tips (1*5 mm. in length) had 
been amputated, they were 
placed vertically in damp 
peat. Of these, three were 
not in the least affected and 
continued for days to grow 
straight downwards. Four 
showed after 8 h. 30 m. a 
mere trace of curvature in 
the direction in which they 
had been acted on by geotro- 
pism; and in this respect 
they differed much from 
those which had been ex- 
posed for 1 h. 37 m., for 
many of the latter were plain- 
ly curved in 6 h. The cur- 
vature of one of these four 
radicles almost disappeared 
after 24 h. In the second, 
the curvature increased dur- 
ing two days and then de- 
creased. The third radicle 




Vidafaba: radicle, rectangularly 
bent at A, after the am])utati(tn 
of the tip, due to the previous 
influence of geotropism. L, side 
of bean which lay on the peat, 
whilst geotropism acted on the 
radicle. A, point of duef curva- 
ture of the radicle, whilst stand- 
ing vertically downwards. 1?, 
point of chief curvature after tlie 
regeneration of the tip, when geot- 
ropism again acted. C, regener- 
ated tip. 
became permanently bent, so 

that its terminal part made an angle of about 45° with its 
original vertical direction. The fourth radicle became horizon- 
tal. These two latter radicles continued during two more days 
to grow in the peat* in the same directions, that is, at an angle 
of 45° beneath the horizon and horizontally. By the IVnntli 
morning new tips had been re-formed, and now geotropism 
was able to act on them again, and they became bent perpen- 
dicularly downwards, exactly as in the case of the live radi- 



532 SENSITIVENESS TO GRAVITATION. Chap. XI. 

cles described in the last paragraph and as is shown in the 
figure (Fig. 195) here given. 

Lastly, five other radicles were similarly treated, but were 
exposed to geotropism during only 45 m. After 8 h. 30 m. only 
one was doubtfully affected; after 24 h. two were just per- 
ceptibly curved tow^ards the side wdiich had been acted on by 
geotropism; after 48 h. the one first mentioned had a radius of 
curvature of 60 mm. That this curvature was due to the action 
of geotropism during the horizontal position of the radicle, was 
shown after 4 days, when a new tip had been re-formed, for it 
then grew perpendicularly downwards. We learn from this 
case that when the tips are amputated after an exposure to ge- 
otropism of only 45 m., though a slight influence is sometimes 
transmitted to the adjoining part of the radicle, yet this seldom 
suffices, and then only slowly, to induce even moderately well- 
pronounced curvature. 

In the previously given experiments on 29 horizontally ex- 
tended radicles with their tips amputated, only one grew^ irregu- 
larly in any marked manner, and this became bowed upwards 
at an angle of 65°. In Ciesielski's experiments the radicles 
could not have grown very irregularly, for if they had done 
so, he could not have spoken confidently of the obliteration 
of all geotropic action. It is therefore remarkable that Sachs, 
who experimented on many radicles with their tips amputated, 
found extremely disordered growth to be the usual result. As 
horizontally extended radicles with amputated tips are some- 
times acted on slightly by geotropism within a short time, and 
are often acted on plainly after one or two days, we thought 
that this influence might possibly prevent disordered growth, 
though it was not able to induce immediate curvature. There- 
fore 13 radicles, of which 6 had their tips amputated trans- 
versely for a length of 1*5 mm., and the other 7 for a length of 
only 0*5 mm., were suspended vertically in damp air, in which 
position they would not be affected by geotropism; but they 
exhibited no great irregularity of growth, whilst observed 
during 4 to 6 days. We next thought that if care were not 
taken in cutting off the tips transversely, one side of the stump 
might be irritated more than the other, either at first or sub- 
sequently during the regeneration of the tip, and that this 



Chap. XI. TRANSMITTED EFFECTS : VICIA. 533 

might cause the radicle to bend to one side. It has also been 
shown in Chapter III. that if a thin slice be cut off one side 
of the tip of the radicle, this causes the radicle to bend from 
the sliced side. Accordingly, 30 radicles, with tips amputated 
for a length of 1*5 mm., were allowed to grow perpendicularly 
downwards into water. Twent}" of them were amputated at an 
angle of 20° with a line transverse to their longitudinal axes; 
and such stumps appeared only moderately oblique. The 
remaining ten radicles were amputated at an angle of about 
45°. Under these circumstances no less than 19 out of the 30 
became much distorted in the course of 3 or 3 days. Eleven 
other radicles were similarly treated, excepting that only 1 mm. 
(including in this and all other cases the root-cap) was ampu- 
tated ; and of these only one grew much and two others sliglitly 
distorted; so that this amount of oblique amputation was not 
sufficient. Out of the above 30 radicles, only one or two showed 
in the first 24 h. any distortion, but this became plain in the 
19 cases on the second day, and still more conspicuous at the 
close of the third day, by which time new tips had been par- 
tially or completely regenerated. When therefore a new tip is 
re-formed on an oblique stump, it probably is developed sooner 
on one side than on the other : and this in some manner excites 
the adjoining part to bend to one side. Hence it seems probable 
that Sachs unintentionally amputated the radicles on which he 
experimented, not strictly in a transverse direction. 

This explanation of the occasional irregular growth of radi- 
cles with amputated tips, is supported by the results of cauter- 
ising their tips; for often a greater length on one side than on 
the other was unavoidably injured or killed. It should be re- 
marked that in the following trials the tips were first dried 
with blotting-paper, and then slightly rubbed with a dried stick 
of nitrate of silver or lunar caustic. A few touches with tlie 
caustic suffice to kill the root-cap and some of the up])er layers 
of cells of the vegetative point. Twenty-seven radicles, some 
young and very short, others of moderate lengtli, were susi)ended 
vertically over water, after being thus cauterised. Of these some 
entered the water immediately, and others on the second day. 
The same number of uncauterised radicles of tlie same age 
were observed as controls. After an interval of three or four 



534 SENSITIVENESS TO GRAVITATION. Chap. XI. 

days the contrast in appearance between the cauterised and 
control specimens was wonderfully great. The controls had 
grown straight downwards, with the exception of the normal 
curvature, which we have called Sachs' curvature. Of the 
27 cauterised radicles, 15 had become extremely distorted; 6 of 
them grew upwards and formed hoops, so that their tips some- 
times came into contact with the bean above; 5 grew out 
rectangularly to one side ; only a few of the remaining 12 were 
quite straight, and some of these towards the close of our 
observations became hooked at their extreme lower ends. 
Radicles, extended horizontally instead of vertically, with their 
tips cauterised, also sometimes grew distorted, but not so com- 
monly, as far as we could judge, as those suspended vertically ; 
for this occurred with only 5 out of 19 radicles thus treated. 

Instead of cutting off the tips, as in the first set of experi- 
ments, we next tried the effects of touching horizontally ex- 
tended radicles with caustic in the manner just described. But 
some preliminary remarks must first be made. It may be ob- 
jected that the caustic would injure the radicles and prevent them 
from bending ; but ample evidence was given in Chapter III. 
that touching the tips of vertically suspended radicles with 
caustic on one side, does not stop their bending; on the 
contrary, it causes them to bend from the touched side. We 
also tried touching both the upper and the lower sides of the 
tips of some radicles of the bean, extended horizontally in damp 
friable earth. The tips of three were touched with caustic on 
their upper sides, and this would aid their geotropic bending; 
the tips of three were touched on their lower sides, which 
would tend to counteract the bending downwards ; and three 
were left as controls. After 24 h. an independent observer was 
asked to pick out of the nine radicles, the two which were most 
and the two which were least bent ; he selected as the latter 
two of those which had been touched on their lower sides, and 
as the most bent, two of those which had been touched on the 
upper side. Hereafter analogous and more striking experiments 
with Pisum sativum and Cucurhita ovifera will be given. We 
may therefore safely conclude that the mere application of 
caustic to the tip does not prevent the radicles from bending. 

In the following exjDeriments, the tips of young horizontally 



Chap. XI. TRANSMITTED EFFECTS: VICIA. 535 

extended radicles were just touched with a stick of dry caustic; 
and this was held transversely^ so that the tip migiit be cau- 
terised all round as sj^mmetrically as possible. The radicles 
were then suspended in a closed vessel over water, kept rather 
cool, viz., 55°-59° F. This was done because we had found 
that the tips were more sensitive to contact under a low than 
under a high temperature; and we thought that the same rule 
might apply to geotropism. In one exceptional trial, nine 
radicles (which were rather too old, for they had grown to a 
length of from 3 to 5 cm.), were extended horizontally in damp 
friable earth, after their tips had been cauterised, and were 
kept at too high a temperature, viz., of 68° F., or 20° C. The 
result in consequence was not so striking as in the subsequent 
cases; for although when after 9 h. 40 m. six of them were 
examined, these did not exhibit any geotropic bending, yet after 
34 h., when all nine were examined, only two remained hori- 
zontal, two exhibited a trace of geotropism, and five were 
slightly or moderately geotropic, yet not comparable in degree 
with the control specimens. Marks had been made on seven of 
these cauterised radicles at 10 mm, from the tips, which includes 
the whole growing portion; and after the 24 h. this part had a 
mean length of 37 mm., so that it had increased to more tiian 
3| times its original length ; but it should be remembered that 
these beans had been exposed to a rather high temperature. 

Nineteen young radicles with cauterised tips were extended 
at different times horizontally over water. In every trial an 
equal number of control specimens were observed. In the first 
trial, the tips of three radicles were lightly touched with the 
caustic for 6 or 7 seconds, which was a longer apjjlication than 
usual. After 23 h. 30 m. (temp. 55°-5G° F.) these three radicles 
A, B, C (Fig. 196), were still horizontal, whilst the three con- 
trol specimens had become within 8 h. slightly geotropic, and 
strongly so (D, E, F) in 23 h. 30 m. A dot had been made on 
all six radicles at 10 mm. from their tips, when first placed 
horizontally. After the 23 h. 30m. this terminal i)art, originally 
10 mm. in length, had increased in the cauterised specimens to 
a mean length of 17 3 mm., and to 15-7 mm. in the control 
radicles, as shown in the figures by the unbroken transverse 
line; the dotted line being at 10 mm. from the apex. The con- 
35 



536 



SENSITIVENESS TO GRAVITATION. Chap. XI. 



trol or uncauterised radicles, therefore, had actually grown less 
than the cauterised; but this no doubt was accidental, for 
radicles of diiierent ages grow at different rates, and the growth 
of different individuals is likewise affected by unknown causes. 
The state of tlie tips of these three radicles, which had been 
cauterised for a rather longer time than usual, was as follows: 

Fig. 196. 




i^^ 



-=€i 



Vicia faba : state of radicles which had been extended horizontally for 
23 h. 30 m. : A, B, C, tips touched with caustic ; D, E, F, tips un- 
cauterised. Lengths of radicles reduced to one-half scale, but by 
an accident the beans themselves not reduced in the same degree. 

the blackened apex, or the part which had been actually touched 
by the caustic, was succeeded by a yellowish zone, due probably 
to the absorption of some of the caustic; in A, both zones 
together were 1"1 mm. in length, and 1*4 mm. in diameter at 
the base of the yellowish zone ; in B, the length of both w^as 
only 0-7 mm., and the diameter 0'7 mm.; in C, the length was 
0-8 mm., and the diameter 1-2 mm. 

Three other radicles, the tips of which had been touched with 
caustic during 2 or 3 seconds, remained (temp. 58°-59° F.) 
horizontal for 23 h. ; the control radicles having, of coarse, 
become geotropic within this time. The terminal growing part, 
10 mm. in length, of the cauterised radicles had increased in 
this interval to a mean length of 24 5 mm., and of the controls 
to a mean of 26 mm. A section of one of the cauterised tips 



:) 



Chap. XI. TRANSMITTED EFFECTS : VICIA. 537 

showed that the blackened part was 05 mm. in length, of which 
0-2 mm. extended into the vegetative point; and a faint dis- 
coloration could be detected even to 1-6 mm. from the apex of 
the root-cap. 

In another lot of six radicles (temp. 55°-57° F.) the three 
control specimens were plainly geotropic in 8.V h- ; find after 24 h. 
the mean length of their terminal part had increased from 
10 mm. to 21 mm. When the caustic was applied to the three 
cauterised specimens, it was held quite motionless during 

5 seconds, and the result was that the black marks were ex- 
tremely minute. Therefore, caustic was again applied, after 
8 J h., during which time no geotropic action had occurred. 
When the specimens were re-examined after an additional 
interval of 15|^ h., one was horizontal and the other two showed, 
to our surprise, a trace of geotropisra which in one of them 
soon afterwards became strongly marked; but in this latter 
specimen the discoloured tip was only f mm. in length. The 
growing part of these three radicles increased in 24 h. from 
10 mm. to an average of 16 "5 mm. 

It would be superfluous to describe in detail the behaviour 
of the 10 remaining cauterised radicles. The corresponding 
control specimens all became geotropic in 8 h. Of the cauterised, 

6 were first looked at after 8 h., and one alone showed a trace 
of geotropism; 4 were first looked at after 14 h., and one alone 
of these was slightly geotropic. After 23-24 h., 5 of the 10 
were still horizontal, 4 slightly, and one decidedly, geotropic. 
After 48 h. some of them became strongly geotropic. The 
cauterised radicles increased greatly in length, but the measure- 
ments are not worth givdng. 

As five of the last-mentioned cauterised radicles liad become 
in 24 h. somewhat geotropic, these (together with three which 
were still horizontal) had their positions reversed, so that their 
tij)s were now^ a little upturned, and they were again touched 
with caustic. After 24 h. they showed no trace of geotro])ism ; 
whereas the eight corresponding control specimens, which had 
likewise been reversed, in which position the ti|)s of several 
pointed to the zenith, all became geotropic; some having passed 
in the 24 h. through an angle of 180°, others through about 
131^, and ethers through only 00°. The eight radicles, which 



538 SENSITIVENESS TO GRAVITATION. Chap. XI. 

had been twice cauterised, were observed for an additional day 
(i.e. for 48 li. after being reversed), and they still showed no 
signs of geotropism. Nevertheless, they continued to grow rap- 
idly; four were measured a4 h. after being reversed, and they 
had in this time increased in length between 8 and 11 mm. ; 
the other four were measured 48 h. after being reversed, and 
these had increased by 20, 18, 23, and 28 mm. 

In coming to a conclusion with respect to the effects of cau- 
terising the tips of these radicles, we should bear in mind, 
firstly, that horizontally extended control radicles were always 
acted on by geotropism, and became somewhat bowed down- 
wards in 8 or 9 h. ; secondly, that the chief seat of the curvature 
lies at a distance of from 3 to 6 mm. from the tip; thirdly, that 
the tip was discoloured by the caustic rarely for more than 
1 mm. in length; fourthly, that the greater number of the cau- 
terised radicles, although subjected to the full influence of 
geotropism during the whole time, remained horizontal for 24 h., 
and some for twice as long; and that those which did become 
bowed were so only in a slight degree ; fifthly, that the cauter- 
ised radicles continued to grow almost, and sometimes quite, 
as well as the uninjured ones along the part which bends most. 
And lastly, that a touch on the tip with caustic, if on one side, 
far from preventing curvature, actually induces it. Bearing all 
these facts in mind, we must infer that under normal conditions 
the geotropic curvature of the root is due to an influence trans- 
mitted from the apex to the adjoining part where the bending 
takes place ; and that when the tip of the root is cauterised it is 
unable to originate the stimulus necessary to produce geotropic 
curvature. 

As we had observed that grease was highly injurious to some 
plants, we determined to try its effects on radicles. When the 
cotyledons of Phalaris and Avena were covered with grease 
along one side, the growth of this side was quite stopped or 
greatly checked, and as the opposite side continued to grow, 
the cotyledons thus treated became bowed towards the greased 
side. This same matter quickly killed the delicate hypocotyls 
and young leaves of certain plants. The grease which we em- 
ployed was made by mixing lamp-black and olive oil to such a 
consistence that it could be laid on in a thick layer. The tips 



Chap. XL TRANSMITTED EFFECTS: PISUM. 539 

of five radicles of the bean were coated with it for a length of 
3 mm., and to our surprise this 2)art increased in length in 23 h. 
to 71 mm.; the thick layer of grease being curiously drawn 
out. It thus could not have checked much, if at all, the growth 
of the terminal part of the radicle. With respect to geotropism, 
the tips of seven horizontally extended radicles were coated for 
a length of 2 mm., and after 24 h. no clear difference could be 
perceived between their downward curvature and that of an 
equal number of control specimens. The tips of 33 other radicles 
were coated on different occasions for a length of 3 mm. ; and 
they were compared with the controls after 8 h., 24 h., and 48 h. 
On one occasion, after 24 h., there was very little difference in 
curvature between the greased and control specimens; but 
generally the difference was unmistakable, those with greased 
tips being considerably less curved downwards. The whole 
growing part (the greased tips included) of six of these radicles 
was measured and was found to have increased in 23 h. from 
10 mm. to a mean length of 17*7 mm. ; whilst the correspond- 
ing part of the controls had increased to 20 "8 mm. It appears 
therefore, that although the tip itself, when greased, con- 
tinues to grow, yet the growth of the whole radicle is some- 
what checked, and that the geotropic curvature of the u])per 
part, which was free from grease, was in most cases consider- 
ably lessened. 

Pisum sativum. — Five radicles, extended horizontally over 
water, had their tips lightly touched two or three times with dry 
caustic. These tips were measured in two cases and found to 
be blackened for a length of only half a millimeter. Five other 
radicles were left as controls. The part which is most bowed 
through geotropism lies at a distance of several millimeters from 
tlie apex. After 24 h., and again after 32 h. from the commence- 
ment, four of the cauterised radicles were still horizontal, hut 
one was plainly geotropic, being inclined at 45° beneath the 
horizon. The five controls were somewhat geotropic after 7 h. 
20 m., and after 24 h. were all strongly geotropic ; being inclined 
at the following angles beneath the horizon, viz., 59°, 00°, 05°, 
57°, and 43°. The length of the radicles was not measured in 
either set, but it was manifest that the cauterised radicles had 
grown greatly. 



540 SENSITIVENESS TO GRAVITATION. Chap. XI. 

The following case proves that the action of the caustic by 
itself does not prevent the curvature of the radicle. Ten radicles 
were extended horizontally on and beneath a layer of damp 
friable peat-earth; and before being extended their tips were 
touched with dry caustic on the upper side. Ten other radicles 
similarly placed were touched on the lower side ; and this would 
tend to make them bend from the cauterised side ; and therefore, 
as now placed, upwards, or in opposition to geotropism. Lastly, 
ten uncauterised radicles were extended horizontally as controls. 
After 24 h. all the latter were geotropic; and the ten with their 
tips cauterised on the upper side were equally geotropic ; and 
we believe that they became curved downwards before the con- 
trols. The ten which had been cauterised on the lower side 
presented a widely different appearance: No 1, however, was 
perpendicularly geotropic, but this was no real exception, for on 
examination under the microscope, there was no vestige of a 
coloured mark on the tip, and it was evident that by a mistake 
it had not been touched with the caustic. No. 2 was plainly 
geotropic, being inclined at about 45*^ beneath the horizon ; No. 3 
was slightly, and No. 4 only just perceptibly geotropic; Nos. 5 
and 6 were strictly horizontal ; and the four remaining ones were 
bowed upwards, in opposition to geotropism. In these four 
cases the radius of the upward curvatures (according to Sachs' 
cyclometer) was 5 mm., 10 mm., 30 mm., and 70 mm. This cur- 
vature was distinct long before the 24 h. had elapsed, namely, 
after 8 h. 45 m. from the time when the lower sides of the tips 
were touched with the caustic. 

PhaseoLus multiflorus. — Eight radicles, serving as controls, 
were extended horizontally, some in damp friable peat and some 
in damp air. They all became (temp. 20°-21° C.) plainly geo- 
tropic in 8 h. 30 m., for they then stood at an average angle of 
63° beneath the horizon. A rather greater length of the radicle is 
bowed downwards by geotropism than in the case of Viciafala, 
that is to say, rather more than 6 mm. as measured from the apex 
of the root-cap. Nine other radicles were similarly extended, 
three in damp peat and six in damp air, and dry caustic was 
held transversely to their tips during 4 or 5 seconds. Three of 
their tips were afterwards examined, in (1) a length of 0*68 mm. 
was discoloured, of which the basal 0-136 mm. was yellow, the 



Chap. XI. TRANSMITTED EFFECTS: PHASEOLUS. 54I 

apical part being black; in (2) the discoloration was 0-65 mm. 
in length, of which the basal 0-04 mm, was yellow ; in (3j the dis- 
coloration was 0-6 mm. in length, of which the basal 0-13 mm. 
was yellow. Therefore less than 1 mm. was affected by the caus- 
tic, but this sufficed almost wholly to prevent geotropic action ; 
for after 24 h. one alone of the nine cauterised radicles became 
slightly geotropic, being now inclined at 10° beneath the horizon ; 
the eight others remained horizontal, though one was curved a 
little laterally. 

The terminal part (10 mm. in length) of the six cauterised 
radicles in the damp air, had more than doubled in length in 
the 24 h., for this part was now on an average 20*7 mm. long. 
The increase in length within the same time was greater in 
the control specimens, for the terminal part had grown on an 
average from 10 mm. to 26-6 mm. But as the cauterised 
radicles had more than doubled their length in the 24 h., it is 
manifest that they had not been seriously injured by tlie 
caustic. We may here add that when experimenting on the 
effects of touching one side of the tip with caustic, too much 
was applied at first, and the whole tip (but we believe not more 
than 1 mm. in length) of six horizontally extended radicles was 
killed, and these continued for two or three days to grow out 
horizontally. 

Many trials were made, by coating the tips of liorizontnlly 
extended radicles with the before described thick grease. The 
geotropic curvature of 12 radicles, which were thus coated for 
a length of 2 mm., was delayed during the first 8 or 9 h., but 
after 24 h. was nearly as great as that of the control specimens. 
The tips of nine radicles were coated for a length of 3 mm., 
and after 7 h. 10 m. these stood at an average angle of 30"^ 
beneath the horizon, whilst the controls stood at an average of 
54°. After 24 h. the two lots differed but little in their degree 
of curvature. In some other trials, however, there was a fairly 
well-marked difference after 24 h. between those with greased 
tips and the controls. The terminal part of eiglit control sjjeci- 
mens increased in 24 h. from 10 mm. to a mean lengtli of 
24*3 mm., whilst the mean increase of those with greased ti|)s 
was 20-7 mm. The grease, therefore, slightly checked tiu> 
growth of tiie terminal part, l)ut this part was not much 



542 SENSITIVENESS TO GRAVITATION. Chap. XI. 

injured ; for several radicles which had been greased for a 
length of 2 mm. continued to grow during seven days^ and were 
then only a little shorter than the controls. The appearance 
presented by these radicles after the seven days was very 
curious, for the black grease had been drawn out into the finest 
longitudinal strise, with dots and reticulations, which covered 
their surfaces for a length of from 26 to 44 mm., or of 1 to 
1-7 inch. We may therefore conclude that grease on the tips 
of the radicles of this Phaseolus somewhat delays and lessens 
the geotropic curvature of the part which ought to bend 
most. 

Gossypium herhaceum. — The radicles of this plant bend, 
through the action of geotropism, for a length of about 6 mm. 
Five radicles, placed horizontally in damp air, had their tips 
touched with caustic, and the discoloration extended for a 
length of from f to 1 mm. They showed, after 7 h. 45 m. and 
again after 23 h., not a trace of geotropism; yet the terminal 
portion, 9 mm. in length, had increased on an average to 
15-9 mm. Six control radicles, after 7 h. 45 m., were all plainly 
geotropic, two of them being vertically dependent, and after 
23 h. all were vertical, or nearly so. 

CucurMta omfera. — A large number of trials proved almost 
useless, from the three following causes: Firstly, the tips of 
radicles which have grown somewhat old are only feebly geo- 
tropic if kept in damp air; nor did we succeed well in our 
experiments, until the germinating seeds were placed in peat 
and kept at a rather high temperature. Secondly, the hypocotyls 
of the seeds which were pinned to the lids of the jars gradually 
became arched; and, as the cotyledons were fixed, the movement 
of the hypocotyl affected the position of the radicle, and caused 
confusion. Thirdly, the point of the radicle is so fine that it is 
difficult not to cauterise it either too much or too little. But 
we managed generally to overcome this latter difficulty, Hs the 
following experiments show, which are given to prove that a 
touch with caustic on one side of the tip does not prevent the 
upper part of the radicle from bending. Ten radicles were laid 
horizontally beneath and on damp friable peat, and their tips 
were touched with caustic on the upper side. After 8 h. all 
were plainly geotropic, three of them rectangularly; after 19 h. 



Chap. XI. TRANSMITTED EFFECTS : CUCURBITA. 543 

all were strongly geotropic, most of them pointing perpen- 
dicularly downwards. Ten other radicles, similarly placed, had 
their tips touched with caustic on the lower side ; after 8 h. 
three were slightly geotropic, but not nearly so much so as the 
least geotropic of the foregoing specimens ; four remained hori- 
zontal ; and three were curved upwards in opposition to geot- 
roplsm. After 19 h. the three which were slightly geotropic 
had become strongly so. Of the four horizontal radicles, one 
alone showed a trace of geotropism; of the three up-curved 
radicles, one retained this curvature, and the other two had 
become horizontal. 

The radicles of this plant, as already remarked, do not suc- 
ceed well in damp air, but tlie result of one trial may be briefly 
given. Nine young radicles between '3 and '5 inch in length, 
with their tips cauterised and blackened for a length never 
exceeding ^ mm., together with eight control specimens, were 
extended horizontally in damp air. After an interval of only 
4 h. 10 m. all the controls were slightly geotropic, whilst not 
one of the cauterised specimens exhibited a trace of this action. 
After 8 h. 35 m., there was the same difference between the 
two sets, but rather more strongly marked. By this time both 
sets had increased greatly in length. The controls, however, 
never became much more curved downwards; and after 24 h. 
there was no great difference between the two sets in their 
degree of curvature. 

Eight young radicles of nearly equal length (average '36 inch) 
were placed beneath and on peat-earth, and were exposed to a 
temp, of 75°-76° F. Their tips had been touched transversely 
with caustic, and five of them were blackened for a lengtli of 
about 0*5 mm., whilst the other three were only just visibly dis- 
coloured. In the same box there were 15 control radicles, mostly 
about '36 incli in length, but some rather longer and older, and 
therefore less sensitive. After 5 h., the 15 control radicles were 
all more or less geotropic: after 9 h., eight of them were bent 
down beneath the horizon at various angles between 45° and 90°, 
the remaining seven being only slightly gt'otropic: after 25 h. 
all were rectangularly geotroj)ic. The state of the eight cauter- 
ised radicles after the same intervals of time was as follows: 
after 5 h. one alone was slightly geotropic, and this was one with 



544: SENSITIVENESS TO GRAVITATION. Chap. XI. 

the tip only a very little discoloured: after 9 h. the one just 
mentioned was rectangularly geotropic and two others were 
slightly so, and these were the three which had been scarcely 
affected by the caustic; the other five were still strictly hori- 
zontal. After 24 h. 40 m. the three with only slightly discol- 
oured tips were bent down rectangularly ; the other five were 
not in the least affected, but several of them had grown rather 
tortuously, though still in a horizontal plane. The eight cauter- 
ised radicles which had at first a mean length of '36 inch, after- 
9 h. had increased to a mean length of -79 inch ; and after 24 h. 
40 m. to the extraordinary mean length of 2 inches. There 
was no plain difference in length between the five well cau- 
terised radicles which remained horizontal, and the three with 
slightly cauterised tips which had become abruptly bent down. 
A few of the control radicles were measured after 25 h., and 
they were on an average only a little longer than the cauterised, 
viz., 2 '19 inches. We thus see that killing the extreme tip of 
the radicle of this plant for a length of about 0'5 mm., though 
it stops the geotropic bending of the upper part, hardly inter- 
feres with the growth of the whole radicle. 

In the same box with the 15 control specimens, the rapid 
geotropic bending and growth of which have just been de- 
scribed, there were six radicles, about -6 inch in length, ex- 
tended horizontally, from which the tips had been cut off in 
a transverse direction for a length of barely 1 mm. These 
radicles were examined after 9 h. and again after 24 h. 40 m., 
and they all remained horizontal. They had not become 
nearly so tortuous as those above described which had been 
cauterised. The radicles with their tips cut off had grown in 
the 24 h. 40 m. as much, judging by the eye, as the cauterised 
specimens. 

Zea mays. — The tips of several radicles, extended horizontally 
in damp air, were dried witli blotting-paper and then touched 
in the first trial during 2 or 3 seconds with dry caustic ; but 
this was too long a contact, for the tips were blackened for a 
length of rather above 1 mm. They showed no signs of geot- 
tropism after an interval of 9 h., and were then thrown away. 
In a second trial the tips of three radicles were touched for a 
shorter time, and were blackened for a length of from 0'5 to 



Chap. XL TRANSMITTED EFFECTS : COXCLUSIOX. 5^,5 

0.75 mm. : they all remained horizontal for 4 h., but after 8 h. 
30 m. one of them, in which the blackened tip was only 0-5 mm. 
in length, was inclined at 21" beneath the horizon. Six con- 
trol radicles all became slightly geotropic in 4 h., and strongly 
so after 8 h. 30 m., with the chief seat of curvature generally 
between 6 or 7 mm. from the apex. In the cauterised specimens, 
the terminal growing part, 10 mm. in length, increased during 
the 8 h. 30 m. to a mean length of 13 mm. ; and in the controls 
to 14 3 mm. 

In a third trial the tips of five radicles (exposed to a temp, 
of 70°-71°) were touched with the caustic only once and very 
slightly ; they were afterwards examined under the microscope, 
and the part which was in any way discoloured was on an 
average '76 mm. in length. After 4 h. 10 m. none were bent; 
after 5 h. 45 m., and again after 23 h. 30 m., they still remained 
horizontal, excepting one which was nov/ inclined 20° beneath 
the horizon. The terminal part, 10 mm. in length, had in- 
creased greatly in length during the 23 h. 30 m., viz., to an 
average of 26 mm. Four control radicles became slightly geo- 
tropic after the 4 h. 10 m., and plainly so after the 5 h. 45 m. 
Their mean length after the 23 h. 30 m. had increased from 
10 mm. to 31 mm. Therefore, a slight cauterisation of the tip 
checks slightly the growth of the whole radicle, and manifestly 
stops the bending of that part which ought to bend most under 
the influence of geotropism and which still continues to 
increase greatly in length. 

Concluding Remarks.— K.hm\(\i\x\t evidence has now 
been given, showing that with various plants the tip 
of the radicle is alone sensitive to geotropism ; and 
that when thus excited, it causes tlie adjoining parts to 
bend. The exact length of the sensitive part seems to 
be somewhat variable, depending in part on the age of 
the radicle; but the destruction of a length of from less 
than 1 to 1-5 mm. (about ^V^h of an inch), in the several 
species observed, generally sufficed to prevent any part of 
the radicle from bending within 24 h., or even for a 
longer pr^riod. The fact of the tip alone being sensitive 



546 SENSITIVENESS TO GRAVITATION. Chap. XI. 

is so remarkable a fact, that we will here give a brief 
summary of the foregoing experiments. The tips were 
cut off 29 horizontally extended radicles of Vicia faba, 
and with a few exceptions they did not become geotropic 
in 22 or 23 h., whilst unmutilated radicles were always 
bowed downwards in 8 or 9 h. It should be borne in 
mind that the mere act of cutting off the tip of a hori- 
zontally extended radicle does not prevent the adjoining 
parts from bending, if the tip has been previously ex- 
posed for an hour or two to the influence of geotropism. 
The tip after amputation is sometimes completely regen- 
erated in three days ; and it is possible that it may be 
able to transmit an impulse to the adjoining parts before 
its complete regeneration. The tips of six radicles of 
CucurMta ovifera were amputated like those of Vicia 
faba; and these radicles showed no signs of geotropism 
in 24 h. ; whereas the control specimens were slightly 
affected in 5 h., and strongly in 9 h. 

With plants belonging to six genera, the tips of the 
radicles were touched transversely with dry caustic ; and 
the injury thus caused rarely extended for a greater length 
than 1 mm., and sometimes to a less distance, as judged 
by even the faintest discoloration. We thought that this 
would be a better method of destroying the vegetative 
point than cutting it off ; for we knew, from many pre- 
vious experiments and from some given in the present 
chapter, that a touch with caustic on one side of the 
apex, far from preventing the adjoining part from bend- 
ing, caused it to bend. In all the following cases, radi- 
cles with uncauterised tips were observed at the same 
time and under similar circumstances, and they became, 
in almost every instance, plainly bowed downwards in 
one-half or one-third of the time during which the cau- 
terised specimens were observed. With Vicia faba 19 
radicles were cauterised ; 12 remained horizontal during 



Chap. XI. TRANSMITTED EFFECTS : CONCLUSION. 547 

23-24 h. ; 6 became slightly and 1 strongly geotropic. 
Eight of these radicles were afterwards reversed, and 
again touched with caustic, and none of tbetn became 
geotropic in 24 h., whilst the reversed control speci- 
mens became strongly bowed downwards within this 
time. With Phum sativum, five radicles had their tips 
touched with caustic, and after 32 h. four were still hori- 
zontal. The control specimens were slightly geotropic 
in 7 h. 20 m., and strongly so in 24 h. Tiie tips of 9 
other radicles of this plant were touched only on the 
lower side, and 6 of them remained horizontal for 24 h., 
or were upturned in opposition to geotropism ; 2 were 
slightly, and 1 plainly geotropic. With Fhaseolus mvlli- 
florus^ 15 radicles were cauterised, and 8 remained liori- 
zontal for 24 h. ; whereas all the controls were plainly 
geotropic in 8 h. 30 m. Of 5 cauterised radicles of Gos- 
sypium herbaceum, 4 remained horizontal for 23 h. and 
1 became slightly geotropic; 6 control radicles were dis- 
tinctly geotropic in 7 h. 45 m. Five radicles of Cucur- 
hita ovifera remained horizontal in peat-earth during 
25 h., and 9 remained so in damp air during 8-3- h. ; 
whilst the controls became slightly geotropic in 4 h. 
10 m. The tips of 10 radicles of this plant were touched 
on their lotuer sides, and 6 of them remained horizon- 
tal or were upturned after 19 h., 1 being slightly and 3 
strongly geotropic. 

Lastly, the tips of several radicles of Vicia faba and 
Phaseolus miiltiflorus were thickly coated witli gi-ease 
for a length of 3 mm. This matter, which is higlily in- 
jurious to most plants, did not kill or stop the growtli of 
the tips, and only slightly lessened the rate of growth of 
the whole radicle; but it generally delayed a little the 
geotropic bending of the upper part. 

The several foregoing cases would tell us nothing, if 
the tip itself was the part which became most bent; but 



548 SENSITIVENESS TO aRAVITATiON. Chap. XI. 

we know that it is a part distant from the tip by some 
millimeters which grows quickest, and which, under the 
influence of geotropism, bends most. We have no reason 
to suppose that this part is injured oy the death or injury 
of the tip ; and it is certain that after the tip has been 
destroyed this part goes on growing at such a rate, that 
its length was often doubled in a day. We have also 
seen that the destruction of the tip does not prevent the 
adjoining part from bending, if this part has already re- 
ceived some influence from the tip. As with horizon- 
tally extended radicles, of which the tip has been cut olf 
or destroyed, the part which ought to bend most remains 
motionless for many hours or days, although exposed at 
right angles to the full influence of geotropism, we must, 
conclude that the tip alone is sensitive to this power, 
and transmits some influence or stimulus to the adjoin- 
ing parts, causing them to bend. We have direct evi- 
dence of such transmission ; for when a radicle was left 
extended horizontally for an hour or an hour and a half, 
by which time the supposed influence will have travelled 
a little distance from the tip, and the tip was then cut 
off, the radicle afterwards became bent, although placed 
perpendicularly. The terminal portions of several radi- 
cles thus treated continued for some time to grow in the 
direction of their newly-acquired curvature ; for as they 
were destitute of tips, they were no longer acted on by 
geotropism. But after three or four days when new vege- 
tative points were formed, the radicles were again acted 
on by geotropism, and now they curved themselves per- 
pendicularly downwards. To see anything of the above 
kind in the animal kingdom, we should have to suppose 
that an animal whilst lying down determined to rise up 
in some particular direction ; and that after its head had 
been cut off, an impulse continued to travel very slowly 
along the nerves to the proper muscles ; so thac after 



Chap. XL TRANSMITTED EFFECTS : CONCLUSION. r>-l-l> 

several hours the headless animal rose up in the prede- 
termined direction. 

As the tip of the radicle has been found to be the 
part which is sensitive to geotropism in the members 
of such distinct families as the Leguminosae, Malvaceae, 
Cucurbitaceae and Gramineae, we may infer that this char- 
acter is common to the roots of most seedling plants. 
Whilst a root is penetrating the ground, the tip must 
travel first ; and we can see the advantage of its being 
sensitive to geotropism, as it has to determine the course 
of the whole root. Whenever the tip is deflected by any 
subterranean obstacle, it will also be an advantage that a 
considerable length of the root should be able to bend, 
more especially as the tip itself grows slowly and bends 
but little, so that the proper downward course may be 
soon recovered. But it appears at first sight immaterial 
whether this were efi'ected by the whole growing part 
being sensitive to geotropism, or by an influence trans- 
mitted exclusively from the tip. We should, however, 
remember that it is the tip which is sensitive to the con- 
tact of hard objects, causing the radicle to bend away 
from them, thus guiding it along the lines of least re- 
sistance in the soil. It is again the tip which is alone 
sensitive, at least in some cases, to moisture, causing the 
radicle to bend towards its source. These two kinds of 
sensitiveness conquer for a time the sensitiveness to geot- 
ropism, which, however, ultimately prevails. Therefore, 
the three kinds of sensitiveness must often come into an- 
tagonism ; first one prevailing, and then another ; and it 
would be an advantage, perhaps a necessity, for the in- 
terweighing and reconciling of these three kinds of sen- 
sitiveness, that they should be all localised in the same 
group of cells which have to transmit the command to 
the adjoining parts of the radicle, causing it to bend to 
or from the source of irritation. 



550 SENSITIVENESS TO GRAVITATION. Chap. XI. 

Finally, the fact of the tip alone being sensitive to 
the attraction of gravity has an important bearing on 
the theory of geotropism. Authors seem generally to 
look at the bending of a radicle towards the centre of 
the earth, as the direct result of gravitation, which is 
believed to modify the growth of the upper or lower sur- 
faces, in such a manner as to induce curvature in the 
proper direction. But we now know that it is the tip 
alone which is acted on, and that this part transmits some 
influence to the adjoining parts, causing them to curve 
downwards. Gravity does not appear to act in a more 
direct manner on a radicle, than it does on any lowly 
organised animal, which moves away when it feels some 
weight or pressure. 



CHAPTER XII. 

Summary and Concluding Remarks. 

Nature of the circumnutatiiig moveroent — History of a germinating 
seed — The radicle first protrudes and circumnutates — Its tip 
highly sensitive — Emergence of the hypocotyl or of the epieotyl 
from the ground under the form of an arch — Its circumnutation 
and that of the cotyledons — The seedling throws up a leaf-bearing 
stem — The circumnutation of all the parts or organs — Modified 
circumnutation — Epinasty and hyponasty — Movements of climb- 
ing plants — Xyctitropic movements — Movements excited by light 
and gravitation — Localised sensitiveness — Resemblance between 
the movements of plants and animals — The tip of the radicle acts 
like a brain. 

It may be useful to the reader if we briefly sum up 
the chief conchisions, which, as far as we can judge, 
have been fairly well established by the observations 
given in this volume. All the parts or organs in every 
plant whilst they continue to grow, and some parts 
which are provided with pulvini after they have ceased 
to grow, are continually circumnutating. This move- 
ment commences even before the young seedling has 
broken through the ground. The nature of the move- 
ment and its causes, as far as ascertained, have been 
briefly described in the Introduction. Why every part 
of a plant whilst it is growing, and in some cases after 
growth has ceased, should have its cells rendered more 
turgescent and its cell-walls more extensile first on one 
side and then on another, thus inducing circumnutation, 
is not known. It would appear as if the changes in the 
cells required periods of rest. 

36 551 



552 SUMMARY AND Chap. XII. 

In some cases, as with the hypocotyls of Brassica, 
the leaves of Dionaea and the joiots of the Gramineae, 
the eircumnutating movement when viewed under the 
microscope is seen to consist of innumerable small oscil- 
lations. The part under observation suddenly jerks 
forwards for a length of "002 to -001 of an inch, and then 
slowly retreats for a part of this distance ; after a few 
seconds it again jerks forwards, but with many intermis- 
sions. The retreating movement apparently is due to the 
elasticity of the resisting tissues. How far this oscilla- 
tory movement is general we do not know, as not many 
eircumnutating plants were observed by us under the 
microscope ; but no such movement could be detected in 
the case of Drosera with a 2-inch object-glass which we 
used. The phenomenon is a remarkable one. The 
whole hypocotyl of a cabbage or the whole leaf of a 
Dionsea could not jerk forwards unless a very large 
number of cells on one side were simultaneously affected. 
Are we to suppose that these cells stea<]ily become more 
and more turgescent on one side, until the part suddenly 
yields and bends, inducing what may be called a micro- 
scopically minute earthquake in the plant ; or do the 
cells on one side suddenly become turgescent in an 
intermittent manner; each forward movement thus 
caused being opposed by the elasticity of the tissues? 

Oircumnutation is of paramount importance in the 
life of every plant; for it is through its modification 
that many highly beneficial or necessary movements 
have been acquired. When light strikes one side of a 
plant, or light changes into darkness, or when gravita- 
tion acts on a displaced part, the plant is enabled in some 
unknown manner to increase the always varying turges- 
cence of the cells on one side ; so that the ordinary 
eircumnutating movement is modified, and the part 
bends either to or from the exciting cause ; or it may 



Chap. Xn. CONCLUDING REMARKS. 553 

occupy a new position, as in the so-called sleep of leaves. 
The influence which modifies circumnutation may 
be transmitted from one part to another. Innate or 
constitutional changes, independently of any external 
agency, often modify the circumnutating movements at 
particular periods of the life of the plant. As circumnu- 
tation is universally present, we can understand how it is 
that movements of the same kind have been developed in 
the most distinct members of the vegetable series. But 
it must not be supposed that all the movements of 
plants arise from modified circumnutation; for, as we 
shall presently see, there is reason to believe that this is 
not the case. 

Having made these few preliminary remarks, we will 
in imagination take a germinating seed, and consider 
the part which the various movements play in the life- 
history of the plant. The first change is the protrusion 
of the radicle, which begins at once to circumnutate. 
This movement is immediately modified by the attraction 
of gravity and rendered geotropic. The radicle, there- 
fore, supposing the seed to be lying on the surface, 
quickly bends downwards, following a more or less spiral 
course, as was seen on the smoked glass-plates. Sensi- 
tiveness to gravitation resides in the tip ; and it is the 
tip which transmits some influence to the adjoining 
parts, causing them to bend. As soon as the tip, pro- 
tected by the root-cap, reaches the ground, it penetrates 
the surface if this be soft or friable ; and the act of 
penetration is apparently aided by the rocking or cir- 
cumnutating movement of the whole end of the radicle. 
If the surface is compact, and cannot easily be pene- 
trated, then the seed itself, unless it be a heavy one, is 
displaced or lifted up by the continued growth and elon- 
gation of the radicle. But in a state of nature seeds 



554 SUMMARY AND Chap. XII. 

often get covered with earth or other matter, or fall into 
crevices, &c., and thus a point of resistance is afforded, 
and the tip can more easily penetrate the ground. 
But even with seeds lying loose on the surface there 
is another aid : a multitude of excessively fine hairs 
are emitted from the upper part of the radicle, and these 
attach themselves firmly to stones or other objects lying 
on the surface, and can do so even to glass ; and thus 
the upper part is held down whilst the tip presses against 
and penetrates the ground. The attachment of the 
root-hairs is effected by the liquefaction of the outer sur- 
face of the cellulose walls, and by the subsequent setting 
hard of the liquefied matter. This curious process prob- 
ably takes place, not for the sake of the attachment of 
the radicles to superficial objects, but in order that the 
hairs may be brought into the closest contact with the 
particles in the soil, by which means they can absorb the 
layer of water surrounding them, together with any dis- 
solved matter. 

After the tip has penetrated the ground to a little 
depth, the increasing thickness of the radicle, together 
with the root-hairs, hold it securely in its place ; and 
now the force exerted by the longitudinal growth of the 
radicle drives the tip deeper into the ground. This 
force, combined with that due to transverse growth, gives 
to the radicle the power of a wedge. Even a growing 
root of moderate size, such as that of a seedling bean, can 
displace a weight of some pounds. It is not probable 
that the tip when buried in compact earth can actually 
circum nutate and thus aid its downward passage, but the 
circumnutating movement will facilitate the tip entering 
any lateral or oblique fissure in the earth, or a burrow 
made by an earth-worm or larva ; and it is certain that 
roots often run down the old burrows of worms. The 
tip, however, iu endeavouring to circumnutate, will con- 



Chap. XII. , CONCLUDING RE.AIARKS. 555 

tinually press against the earth on all sides, and this can 
hardly fail to be of the highest importance to the plant ; 
for we have seen that when little bits of card-like paper 
and of very thin paper were cemented on opposite sides 
of the tip, the whole growing part of the radicle was ex- 
cited to bend away from the side bearing the card or 
more resisting substance, towards the side bearing the 
thin paper. We may therefore feel almost sure that 
when the tip encounters a stone or other obstacle in the 
ground, or even earth more compact on one side than the 
other, the root will bend away as much as it can from 
the obstacle or the more resisting earth, and will thus 
follow with unerring skill a line of least resistance. 

The tip is more sensitive to prolonged contact with 
an object than to gravitation when this acts obliquely 
on the radicle, and sometimes even when it acts in the 
most favourable direction at right angles to the radicle. 
The tip was excited by an attached bead of shellac, 
weighing less than ^^^-th of a grain (0-33 mg.) ; it is there- 
fore more sensitive than the most delicate tendril, name- 
ly, that of Passifiora gracilis^ which was barely acted on 
by a bit of wire weighing -^-^ih. of a grain. But this de- 
gree of sensitiveness is as nothing compared with that of 
the glands of Drosera, for these are excited by particles 
weighing only Ys^ro ^^ ^ grain. The sensitiveness of the 
tip cannot be accounted for by its being covered by a 
thinner layer of tissue than the other parts, for it is pro- 
tected by the relatively thick root-cap. It is remarkable 
that although the radicle bends away, when one side of 
the tip is slightly touched with caustic, yet if the side be 
much cauterised the injury is too great, and the power 
of transmitting some influence to the adjoining parts 
causing them to bend, is lost. Other analogous cases are 
known to occur. 

After a radicle \\i& been deflected by some obstacle, 



556 SUMMARY AND Chap. XH. 

geotropism directs the tip again to grow perpendicularly 
downwards ; but geotropism is a feeble power, and here, 
as Sachs has shown, another interesting adaptive move- 
ment comes into play ; for radicles at a distance of a few 
millimetres from the tip are sensitive to prolonged con- 
tact in such a manner that they bend towards the touch- 
ing object, instead of from it as occurs when an object 
touches one side of the tip. Moreover, the curvature 
thus caused is abrupt ; the pressed part alone bending. 
Even slight pressure suffices, such as a bit of card ce- 
mented to one side. Therefore a radicle, as it passes 
over the edge of any obstacle in the ground, will through 
the action of geotropism press against it ; and this pres- 
sure will cause the radicle to endeavour to bend abruptly 
over the edge. It will thus recover as quickly as possi- 
ble its normal downward course. 

Eadicles are also sensitive to air which contains more 
moisture on one side than the other, and they bend 
towards its source. It is therefore probable that they are 
in like manner sensitive to dampness in the soil. It was 
ascertained in several cases that this sensitiveness resides 
in the tip, which transmits an influence causing the ad- 
joining upper part to bend in opposition to geotropism 
towards the moist object. We may therefore infer that 
roots will be deflected from their downward course 
towards any source of moisture in the soil. 

Again, most or all radicles are slightly sensitive to 
light, and, according to Wiesner, generally bend a little 
from it. Whether this can be of any service to them is 
very doubtful, but with seeds germinating on the surface 
it will slightly aid geotropism in directing the radicles 
to the ground.* We ascertained in one instance that 



*Dr. Karl Eichter, who has in Wien,' 1879, p. 149), states that 
especially attended to this subject apheliotropism does not aid radi- 
( ' K. Akad. der Wissenschaften cles in penetrating the ground. 



Chap. XII. CONCLUDING REMARKS. 557 

such sensitiveness resided in the tip, and caused the ad- 
joining parts to bend from the light. The sub-aerial 
roots observed by Wiesner were all apheliotropic, and this, 
no doubt, is of use in bringing them into contact with 
trunks of trees or surfaces of rock, as is their habit. 

We thus see that w4th seedling plants the tip of the 
radicle is endowed with diverse kinds of sensitiveness ; 
and that the tip directs the adjoining growing parts to 
bend to or from the exciting cause, according to the 
needs of the plant. The sides of the radicle are also 
sensitive to contact, but in a widely different manner. 
Gravitation, though a less powerful cause of movement 
than the other above specified stimuli, is ever present ; 
so that it ultimately prevails and determines the down- 
ward growth of the root. 

The primary radicle emits secondary ones which pro- 
ject sub-horizontally ; and these were observed in one 
case to circumnutate. Their tips are also sensitive to 
contact, and they are thus excited to bend away from 
any touching object ; so that they resemble in these 
respects, as far, as they were observed, the primary radi- 
cles. If displaced they resume, as Sachs has shown, 
their original sub-horizontal position ; and this appar- 
ently is due to diageotropism. The secondary radicles 
emit tertiary ones, but these, in the case of the bean, are 
not affected by gravitation ; consequently they protrude 
in all directions. Thus the general arrangement of the 
three orders of roots is excellently adapted for searching 
the whole soil for nutriment. 

Sachs has shown that if the tip of the primary radi- 
cle is cut off (and the tip will occasionally be gnawed oil 
with seedlings in a state of nature) one of the secondary 
radicles grows perpendicularly downwards, in a manner 
which is analo.i^ous to the upward growth of a lateral 
shoot after the amputation of the leading shoot. We 



558 SUMMARY AND Chap. XII. 

have seen with radicles of the bean that if the primary 
radicle is merely compressed instead of being cut off, so 
that an excess of sap is directed into the secondary radi- 
cles, their natural condition is disturbed and they grow 
downwards. Other analogous facts have been given. 
As anything which disturbs the constitution is apt to- 
lead to reversion, that is, to the resumption of a former 
character, it appears probable that when secondary radi- 
cles grow downwards or lateral shoots upwards, they 
revert to the primary manner of growth proper to radi- 
cles and shoots. 

With dicotyledonous seeds, after the protrusion of the 
radicle, the hypocotyl breaks through the seed-coats; but 
if the cotyledons are hypogean, it is the epicotyl which 
breaks forth. These organs are at first invariably arched, 
with the upper part bent back parallel to the lower ; and 
they retain this form until they have risen above the 
ground. In some cases, however, it is the petioles of the 
cotyledons or of the first true leaves which break through 
the seed-coats as well as the ground, before any part of 
the stem protrudes ; and then the petioles are almost in- 
variably arched. We have met with only one exception, 
and that only a partial one, namely, with the petioles of 
the two first leaves of Acanthus candelabrum. With 
Delphinium nudicaule the petioles of the two cotyledons 
are completely confluent, and they break through the 
ground as an arch ; afterwards the petioles of the suc- 
cessively formed early leaves are arched, and they are 
thus enabled to break through the base of the confluent 
petioles of the cotyledons. In the case of Megarrhiza, it 
is the plumule which breaks as an arch through the tube 
formed by the confluence of the cotjdedon-petioles. With 
mature plants, the flower-stems and the leaves of some 
few species, and the rachis of several ferns, as they 
emerge separately from the ground, are likewise arched. 



Chap. XII. CONCLUDING REMARKS. 559 

The fact of so many different organs in plants of 
many kinds breaking through the ground under the form 
of an arch, shows that this must be in some manner 
highly important to them. According to Haberlandt, 
the tender growing apex is thus saved from abr-usion, and 
this is probably the true explanation. But as both legs 
of the arch grow, their power of breaking through the 
ground will be much increased as long as the tip remains 
within the seed-coats and has a point of support. In the 
case of monocotyledons the plumule or cotyledon is rare- 
ly arched, as far as we have seen ; but this is the case 
with the leaf-like cotyledon of the onioa ; and the crown 
of the arch is here strengthed by a special protuberance. 
In the Graminese the summit of the straight, sheath-like 
cotyledon is developed into a hard sharp crest, which evi- 
dently serves for breaking through the earth. With di- 
cotyledons the arching of the epicotyl or hypocotyl often 
appears as if it merely resulted from the manner in which 
the parts are packed within the seed ; but it is doubtful 
whether this is the whole of the truth in any case, and it 
certainly was not so in several cases, in which the arch- 
ing was seen to commence after the parts had wholly 
escaped from the seed-coats. As the arching occurred 
in whatever position the seeds were placed, it is no doubt 
due to temporarily increased growth of the nature of 
epinasty or hyponasty along one side of the part. 

As this habit of the hypocotyl to arch itself appears 
to be universal, it is probably of very ancient origin. It 
is therefore not surprising that it should be inherited, at 
least to some extent, by plants having hypogean cotyle- 
dons, in which the hypocotyl is only slightly developed 
and never protrudes above the ground, and in which the 
arching is of course now quite useless. This tendency 
explains, as we have seen, the curvature of tlie hypocotyl 
(and the consequent movement of the radich') which was 



560 SUMMARY AND Chap. XII. 

first observed by Sachs, and which we have often had to 
refer to as Sachs' curvature. 

The several foregoing arched organs are continnally 
circumnntating, or endeavouring to circumnutate, even 
before they break through the ground. As soon as any 
part of the arch protrudes from the seed-coats it is acted 
upon by apogeotropisni, and both the legs bend upwards 
as quickly as the surrounding earth will permit, until the 
arch stands vertically. By continued growth it then 
forcibly breaks through the ground ; but as it is contin- 
ually striving to circumnutate this will aid its emergence 
in some slight degree, for we know that a circumnntating 
hypocotyl can push away damp sand on all sides. As 
soon as the faintest ray of light reaches a seedling, heli- 
otropism will guide it through any crack in the soil, or 
through an entangled mass of overlying vegetation ; for 
apogeotropisni by itself can direct the seedling only blind- 
ly upwards. Hence probably it is that sensitiveness to 
light resides in the tip of the cotyledons of the Gramineae, 
and in the upper part of the hypocotyls of at least some 
plants. 

As the arch grows upwards the cotyledons are dragged 
out of the ground. The seed-coats are either left behind 
buried, or are retained for a time still enclosing the coty- 
ledons. These are afterwards cast off merely by the 
swelling of the cotyledons. But with most of the Cu- 
curbitacese there is a curious special contrivance for burst- 
ing the seed-coats whilst beneath the ground, namely, a 
peg at the base of the hypocotyl, projecting at right 
angles, which holds down the lower half of the seed- 
coats, whilst the growth of the arched part of the hypo- 
cotyl lifts up the upper half, and thus splits them in 
twain. A somewhat analogous structure occurs in Mi- 
7nosa piuUca and some other plants. Before the cotyle- 
dons are fully expanded and have diverged, the hypocotyl 



CuAP. XII. CONCLUDING REMARKS. 561 

generally straightens itself by increased growth along the 
concave side, thus reversing the process which caused the 
arching. Ultimately not a trace of the former curvature 
is left, except in the case of the leaf-like cotyledons of 
the onion. 

The cotyledons can now assume the function of leaves, 
and decompose carbonic acid ; they also yield up to other 
parts of the plant the nutriment which they often con- 
tain. When they contain a large stock of nutriment they 
generally remain buried beneath the ground, owing to 
the small development of the hypocotyl ; and thus they 
have a better chance of escaping destruction by animals. 
From unknown causes, nutriment is sometimes stored in 
^the hypocotyl or in the radicle, and tlien one of the coty- 
ledons or both become rudimentary, of which several in- 
stances have been given. It is probable that the extraor- 
dinary manner of germination of Megarrldza Californica^ 
Ipomcea leptophylla and pandurata, and of Quercns vi- 
rens^ is connected with the burying of the tuber-like 
roots, which at an early age are stocked with nutriment ; 
for in these plants it is the petioles of the cotyledons 
which first protrude from the seeds, and they are then 
merely tipped with a minute radicle and hypocotyl. 
These petioles bend down geotropically like a root and 
penetrate the ground, so that the true root, which after- 
wards becomes greatly enlarged, is buried at some little 
depth beneath the surface. Gradations of structure are 
always interesting, and Asa Gray informs us that with 
Ipornma Jalappa^ which likewise forms huge tubers, the 
hypocotyl is still of considerable length, and the i)etioles 
of the cotyledons are only moderately elongated. But 
in addition to the advantage gained by the concealment 
of the nutritious matter stored within the tubers, the 
plumule, at least in the case of Megarrliiza, is protected 
from the frosts of winter by being buried. 



562 SUMMARY AND Chap. XII. 

With many dicotyledonous seedlings, as has lately 
been described by De Vries, the contraction of the pa- 
renchyma of the upper part of the radicle drags the 
hypocotyl downwards into the earth ; sometimes (it is 
said) until even the cotyledons are buried. The hypo- 
cotyl itself of some species contracts in a like manner. 
It is believed that this burying process serves to protect 
the seedlings against the frosts of winter. 

Our imaginary seedling is now mature as a seedling, 
for its hypocotyl is straight and its cotyledons are fully 
expanded. In this state the upper part of the hypocotyl 
and the cotyledons continue for some time to circumnu- 
tate, generally to a wide extent relatively to the size of 
the parts, and at a rapid rate. But seedlings profU by 
this power of movement only when it is modified, espe- 
cially by the action of light and gravitation ; for they 
are thus enabled to move more rapidly and to a greater 
extent than can most mature plants. Seedlings are sub- 
jected to a severe struggle for life, and it appears to be 
highly important to them that they should adapt them- 
selves as quickly and as perfectly as possible to their con- 
ditions. Hence also it is that they are so extremely sen- 
sitive to light and gravitation. The cotyledons of some 
few species are sensitive to a touch ; but it is probable 
that this is only an indirect result of the foregoing kinds 
of sensitiveness, for there is no reason to believe that they 
profit by moving when touched. 

Our seedling now throws up a stem bearing leaves, 
and often branches, all of which whilst young are con- 
tinually circumnutating. If we look, for instance, at a 
great acacia tree, we may feel assured that every one of 
the innumerable growing shoots is constantly describing 
small ellipses ; as is each petiole, sub-petiole, and leaflet. 
The latter, as well as ordinary leaves, generally move up 
and down in nearly the same vertical plane, so that they 



Chap. XII. CONCLUDING REMARKS. 5G3 

describe very narrow ellipses. The flower-peduncles are 
likewise continLially circumnutating. If we could look 
beneath the ground, and our eyes bad the power of a 
microscope, we sliould see the tip of each rootlet en- 
deavouring to sweep small ellipses or circles, as far as the 
pressure of the surrounding earth permitted. All tliis 
astonishing amount of movement has been going on year 
after year since the time when, as a seedling, the tree 
first emerged from the ground. 

Stems are sometimes developed into long runners or 
stolons. These circumnutate in a conspicuous manner, 
and are thus aided in passing between and over surround- 
ing obstacles. But whether the circumnutating move- 
ment has been increased for this special purpose is 
doubtful. 

We have now to consider circumnutation in a modi- 
fied form, as the source of several great classes of move- 
ment. The modification may be determined by innate 
causes, or by external agencies. Under the first head we 
see leaves which, when first unfolded, stand in a vertical 
position, and gradually bend downwards as they grow 
older. We see flower-peduncles bending down after the 
flower has Avithered, and others rising up ; or again, stems 
with their tips at first bowed downwards, so as to be 
hooked, afterwards straightening themselves; and many 
other such cases. These changes of position, which are 
due to epinasty or hyponasty, occur at certain periods of 
the life of the plant, and are independent of any external 
agency. They are effected not by a continuous upward 
or downward movement, but by a succession of small 
ellipses, or by zigzag lines, — that is, by a circumnutating 
movement which is preponderant in some one direction. 

Again, climbing plants whilst young circumnutate in 
the ordinary manner, but as soon as the stem has grown 
to a certain height, which is different for dilTerent species, 



564 SUMMARY AND Chap. XII. 

it elongates rapidly, and now the amplitude of the circum- 
nutating movement is immensely increased, evidently to 
favour the stem catching hold of a support. The stem 
also circumnutates rather more equally to all sides than 
in the case of non-climbing plants. This is conspicu- 
ously the case with those tendrils which consist of modi- 
fied leaves, as these sweep wide circles ; whilst ordinary 
leaves usually circumnutate nearly in the same vertical 
plane. Flower-peduncles when converted into tendrils 
have their circumnutating movement in like manner 
greatly increased. 

We now come to our second group of circumnutat- 
ing movements — those modified through external agen- 
cies. The so-called sleep or nycti tropic movements of 
leaves are determined by the daily alternations of light 
and darkness. It is not the darkness which excites them 
to move, but the difference in the amount of light which 
they receive during the day and night ; for with several 
species, if the leaves have not been brightly illuminated 
during the day, they do not sleep at night. They in- 
herit, however, some tendency to move at the proper 
periods, independently of any change in the amount of 
light. The movements are in some cases extraordinarily 
complex, but as a full summary has been given in the 
chapter devoted to this subject, we will here say but little 
on this head. Leaves and cotyledons assume their noc- 
turnal position by two means, by the aid of pulvini and 
without such aid. In the former case the movement con- 
tinues as long as the leaf or cotyledon remains in full 
health ; whilst in the latter case it continues only whilst 
the part is growing. Cotyledons appear to sleep in a 
larger proportional number of species than do leaves. 
In some species, the leaves sleep and not the cotyledons ; 
in others, the cotyledons and not the leaves ; or both may 
sleep, and yet assume widely different positions at night. 



Chap. XII. CONCLUDING REMARKS. 565 

Although the nyctitropic movements of leaves and 
cotyledons are wonderfully diversified, and sometimes 
differ much in the species of the same genus, yet the 
blade is always placed in such a position at niglit, that its 
upper surface is exposed as little as possible to full radia- 
tion. We cannot doubt that this is the object gained by 
these movements ; and it has been proved that leaves ex- 
posed to a clear sky, with their blades compelled to re- 
main horizontal, suffered much more from the cold than 
others which were allowed to assume their proper verti- 
cal position. Some curious facts have been given under 
this head, showing that horizontally extended leaves suf- 
fered more at night, when the air, which is not cooled by 
radiation, was prevented from freely circuhiting beneath 
their lower surfaces ; and so it was, when the leaves were 
allowed to go to sleep on branches which had been ren- 
dered motionless. In some species the petioles rise up 
greatly at night, and the pinnae close together. The 
whole plant is thus rendered more compact, and a much 
smaller surface is exposed to radiation. 

That the various nyctitropic movements of leaves 
result from modified circumnutation has, we think, been 
clearly shown. In the simplest cases a leaf describes a 
simple large ellipse during the 24 h. ; and the movement 
is so arranged that the blade stands vertically during the 
night, and roassumes its former position on the following 
morning. The course pursued differs from ordinary cir- 
cumnutation only in its greater amplitude, and in its 
greater rapidity late in the evening and early on the fol- 
lowing morning. Unless this movement is admitted to be 
one of circumnutation, such leaves do not circumnutate 
at all, and this would be a monstrous anomaly. In other 
cases, leaves and cotyledons describe several vertical ellip- 
ses during the 24 h. ; and in the evening one of them is 
increased greatly in amplitude until the blade stands 



566 SUMMARY AND Chap. XII. 

vertically either upwards or downwards. In this posi- 
tion it continues to circumnutate until the following 
morning, when it reassumes its former position. These 
movements, when a pulvinus is present, are often compli- 
cated by the rotation of the leaf or leaflet; and such 
rotation on a small scale occurs during ordinary circum- 
nutation. The many diagrams showing the movements 
of sleeping and non-sleeping leaves and cotyledons should 
be compared, and it will be seen that they are essentially 
alike. Ordinary circumnutation is converted into a nyc- 
titropic movement, firstly by an increase in its amplitude^ 
but not to so great a degree as in the case of climbing 
plants, and secondly by its being rendered periodic in 
relation to the alternations of day and night. But there 
is frequently a distinct trace of periodicity in the cir- 
cumnutating movements of non-sleeping leaves and coty- 
ledons. The fact that nyctitropic movements occur in 
species distributed in many families throughout the 
whole vascular series, is intelligible, if they result from 
the modification of the universally present movement of 
circumnutation ; otherwise the fact is inexplicable. 

In the seventh chapter we have given the case of a 
Porlieria, the leaflets of which remained closed all day, 
as if asleep, when the plant was kept dry, apparently for 
the sake of checking evaporation. Something of the 
same kind occurs with certain Gramineae. At the close 
of this same chapter, a few observations were appended 
on what may be called the embryology of leaves. The 
leaves produced by young shoots on cut-down plants of 
MeUhtus taurica slept like those of a Trifolium, whilst 
the leaves on the older branches on the same plants slept 
in a very different manner, proper to the genus; and 
from the reasons assigned we are tempted to look at this 
case as one of reversion to a former nyctitropic habit. 
So again with Desmodium gyrans, the absence of small 



CiiAP. XII. CONCLUDING REMARKS. 507 

lateral leaflets on very young plants, makes us suspect 
that the immediate progenitor of this species did not 
possess lateral leaflets, and that their appearance in an 
almost rudimentary condition at a somewhat more ad- 
vanced age is the result of reversion to a trifoliate pre- 
decessor. However this may be, the rapid circumnutat- 
ing or gyrating movements of the little lateral leaflets, 
seem to be due proximately to the pulvinus, or organ of 
movement, not having been reduced nearly so much as 
the blade, during the successive modifications through 
which the species has passed. 

We now come to the highly important class of move- 
ments due to the action of a lateral light. When stems, 
leaves, or other organs are placed, so that one side is 
illuminated more brightly than the other, they bend 
towards the light. This heliotropic movement mani- 
festly results from the modification of ordinary circum- 
nutation ; and every gradation between the two move- 
ments could be followed. When the light was dim, and 
only a very little brighter on one side than on the other, 
the movement consisted of a succession of ellipses, di- 
rected towards the light, each of which approached 
nearer to its source than the previous one. When the 
difference in the light on the two sides was somewhat 
greater, the ellipses were drawn out into a strongly- 
marked zigzag line, and when much greater the course 
became rectilinear. AVe have reason to believe that 
changes in the turgescence of the cells is the proximate 
cause of the movement of circumnutation ; and it ap- 
pears that when a plant is unequally illuminated on the 
two sides, the always changing turgescence is augmented 
along one side, and is weakened or quite arrested along 
the other sides. Increased turgescence is commonly fol- 
lowed by increased growth, so that a plant which has 
bent itself towards the light during the day would bo 
87 



568 SUMMARY AND Chap. XII. 

fixed in this position were it not for apogeotropism act- 
ing during the night. But parts provided with puhini 
bend, as Pfeifer has shown, towards the light; and here 
growth does not come into play any more than in the 
ordinary circumnutating movements of pulvini. 

Heliotropism prevails widely throughout the vegetable 
kingdom, but whenever, from the changed habits of life 
of any plant, such movements become injurious or use- 
less, the tendency is easily elimiuated, as we see with 
climbing and insectivorous plants. 

Apheliotropic movements are comparatively rare in a 
well-marked degree, excepting with sub-aerial roots. In 
the two cases investigated by us, the movement certainly 
consisted of modified circumnutation. 

The position which leaves and cotyledons occupy 
during the day, namely, more or less transversely to the 
direction of the light, is due, according to Frank, to 
what we call diaheliotropism. As all leaves and coty- 
ledons are continually circumnutating, there can hardly 
be a doubt that diaheliotropism results from modified 
circumnutation. From the fact of leaves and cotyledons 
frequently rising a little in the evening, it appears as if 
diaheliotropism had to conquer during the middle of the 
day a widely prevalent tendency to apogeotropism. 

Lastly, the leafiets and cotyledons of some plants are 
known to be injured by too much light ; and when the 
sun shines brightly on them, they move upwards or 
downwards, or twist laterally, so that they direct their 
edges towards the light, and thus they escape being in- 
jured. These paraheliotropic movements certainly con- 
sisted in one case of modified circumnutation ; and so it 
probably is in all cases, for the leaves of all the species 
described circumnutate in a conspicuous manner. This 
movement has hitherto been observed only with leaflets 
provided with pulviui, in which the increased turges- 



Chap. XII. CONCLUDING REMARKS. 5(j9 

cence ou opposite sides is not followed by growth ; and 
we can understand why this should be so, as the move- 
ment is required only for a temporary purpose. It would 
manifestly be disadvantageous for the leaf to be fixed by 
growth in its inclined position. For it has to assume its 
former horizontal position, as soon as possible after the 
sun has ceased shining too brightly on it. 

The extreme sensitiveness of certain seedlings to 
light, as shown in our ninth chapter, is highly remark- 
able. The cotyledons of Phalaris became curved to- 
wai-ds a distant lamp, which emitted so little light, 
that a pencil held vertically close to the plants, did not 
cast any shadow which the eye could perceive on a white 
card. These cotyledons, therefore, were affected by a 
difference in the amount of light on their two sides, 
which the eye could not distinguish. The degree of 
their curvnture within a given time towards a lateral 
light did not correspond at all strictly with the amount 
of light which they received ; the light not being at any 
time in excess. They continued for nearly half an hour 
to bend towards a lateral light, after it had been extin- 
guished. They bend with remarkable precision towards 
it, and this depends on the illumination of one whole 
side, or on the obscuration of the whole opposite side. 
The difference in the amount of light which plants at 
any time receive in comparison with what they have 
shortly before received, seems in all cases to be the chief 
exciting cause of those movements which are influenced 
by light. Thus seedlings brought out of darkness bend 
towards a dim lateral light, sooner than others which 
had previously been exposed to daylight. AVe have seen 
several analogous cases with the nyctitropic movements 
of leaves. A striking instance was observed in the case 
of the periodic movements of the cotyledons of a Cassia ; 
in the morning a pot was placed in an obscure part of u 



570 ~ SUMMARY AXD Chap. XII. 

room, and all the cotyledons rose up closed ; another pot 
had stood in the sunlight, and the cotyledons of course 
remained expanded ; both pots were now placed close 
together in the middle of the room, and. the cotyledons 
which had been exposed to the sun, immediately began 
to close, while the others opened ; so that the cotyledons 
in the two pots moved in exactly opposite directions 
whilst exposed to the same degree of light. 

We found that if seedlings, kept in a dark place, 
were laterally illuminated by a small wax taper for only 
two or three minutes at intervals of about three-quarters 
of an hour, they all became bowed to the point where 
the taper had been held. We felt much surprised at this 
fact, and until we had read Wiesner's observations, we 
attributed it to the after-effects of the light ; but he has 
shown that the same degree of curvature in a plant may 
be induced in the course of an hour by several inter- 
rupted illuminations lasting altogether for 20 m., as by 
a continuous illumination of 60 m. We believe that this 
case, as well as our own, may be explained by the ex- 
citement from light being due not so much to its actual 
amount, as to the difference in amount from that pre- 
viously received ; and in our case there were repeated 
alternations from complete darkness to light. In this, 
and in several of the above specified respects, light seems 
to act on the tissues of plants, almost in the same manner 
as it does on the nervous system of animals. 

There is a much more striking analogy of the same 
kind, in the sensitiveness to light being localised in the 
tips of the cotyledons of Phalaris and Avena, and in the 
upper part of the hypocotyls of Brassica and Beta ; and 
in the transmission of some influence from these upper 
to the lower parts, causing the latter to bend towards the 
light. This influence is also transmitted beneath the 
soil to a depth where no light enters. It follows from 



Chap. XII. CONCLUDING REMARKS. 571 

this localisation, that the lower parts of the cotyledons 
of Phalaris, &c., which normally become more bent 
towards a lateral light than the upper parts, may be 
brightly illuminated during many hours, and will not 
bend in the least, if all light be excluded from the tip. It 
is an interesting experiment to place caps over the tips 
of the cotyledons of Plialaris, and to allow very little 
light to enter through minute orifices on one side of the 
caps, for the lower part of the cotyledons will then bend 
to this side, and not to the side which has been brightly 
illuminated during the whole time. In the case of the 
radicles of Sinapis alba, sensitiveness to light also resides 
in the tip, which, when laterally illuminated, causes the 
adjoining part of the root to bend apheliotropically. 

Gravitation excites plants to bend away from the 
centre of the earth, or towards it, or to place themselves 
in a transverse position with respect to it. Although it 
is impossible to modify in any direct manner the attrac- 
tion of gravity, yet its influence could be moderated in- 
directly, in the several ways described in the tenth chap- 
ter; and under such circumstances the same kind of 
evidence as that given in the chapter on Heliotropism, 
showed in the plainest manner that apogeotropic and 
geotropic, and probably diageotropic movements, are all 
modified forms of circumnutation. 

Different parts of the same plant and different species 
are affected by gravitation in widely different degrees 
and manners. Some plants and organs exhibit hardly 
a trace of its action. Young seedlings which, as we 
know, circumnutate rapidly, are eminently sensitive; 
and we have seen the hypocotyl of Beta bending upwards 
through 109° in 3 h. 8 m. The after-effects of apogo- 
otropism last for above half an hour; and horizontally- 
laid hypocotyls are sometimes thus carried temporarily 
beyond an upright position. The benefits derived from 



572 SUMMARY AND Cuap. XII. 

geotropism, apogeotropism, and diageotropism, are gen- 
erally so manifest that they need not be specified. With 
the flower-peduncles of Oxalis, epinasty causes them to 
bend down, so that the ripening pods may be protected 
by the calyx from the rain. Afterwards they are carried 
upwards by apogeotropism in combination with hypo- 
nasty, and are thus enabled to scatter their seeds over a 
wider space. The capsules and flower-heads of some 
plants are bowed downwards through geotropism, and 
they then bury themselves in the earth for the protection 
and slow maturation of the seeds. This burying process 
is much facilitated by the rocking movement due to 
circumnutation. 

In the case of the radicles of several, probably of all 
seedling plants, sensitiveness to gravitation is confined 
to the tip, which transmits an influence to the adjoining 
upper part, causing it to bend towards the centre of the 
earth. That there is transmission of this kind was 
proved in an interesting manner when horizontally ex- 
tended radicles of the bean were exposed to the attraction 
of gravity for 1 or 1-^ h., and their tips were then ampu- 
tated. Within this time no trace of curvature was 
exhibited, and the radicles were now placed pointing ver- 
tically downwards; but an influence had already been 
transmitted from the tip to the adjoining part, for it 
soon became bent to one side, in the same manner as 
would have occurred had the radicle remained horizontal 
and been still acted on by geotropism. Eadicles thus 
treated continued to grow out horizontally for two or 
three days, until a new tip was reformed ; and this was 
then acted on by geotropism, and the radicle became 
curved perpendicularly downwards. 

It has now been shown that the following important 
classes of movement all arise from modified circumnu- 



Chap. XII. CONCLUDING REMARKS. 573 

tation, which is omnipresent whilst growth lasts, and after 
growth has ceased, whenever pulvini are present. These 
classes of movement consist of those due to epinasty and 
hyponasty, — those ^^roper to climbing plants, commonlv 
called revolving nutation, — the nyctitropic or sleep move- 
ments of leaves and cotyledons, — and the two immense 
classes of movements excited by light and gravitation. 
When we speak of modified circumnutation we mean 
that light, or the alternations of light and darkness, 
gravitation, slight pressure or other irritants, and certain 
innate or constitutional states of the plant, do not direct- 
ly cause the movement ; they merely lead to a temporary 
increase or diminution of those spontaneous changes in 
the turgescence of the cells which are already in progress. 
In what manner, light, gravitation, &c., act on the cells 
is not known ; and we will here only remark that, if any 
stimulus affected the cells in such a manner as to cause 
some slight tendency in the affected part to bend in a 
beneficial manner^ this tendency might easily be increased 
through the preservation of the more sensitive individu- 
als. But if such bending were injurious, the tendency 
would be eliminated unless it was overpoweringly strong ; 
for we know how commonly all characters in all organ- 
isms vary. Nor can we see any reason to doubt, that 
after the complete elimination of a tendency to bend in 
some one direction under a certain stimulus, the power 
to bend in a directly opposite direction might gradually 
be acquired through natural selection.* 

Although so many movements have arisen through 
modified circumnutation, there are others which a})pear 
to have had a quite independent origin ; but they do not 
form such large and important classes. When a leaf of 



* See the remarks in Frank's 91, &c.), on natuni! sclt-ttittn in 
'Die wagcreclite Kiclitung von (•(Mnu-ction with K<^'otropisni, heli- 
Pftanzenthc'iU'n ' (1870, j)}). 90. o1r(.i)isiii, &c. 



574 SUMMARY AND Chap. XII. 

a Mimosa is touched it suddenly assumes the same posi- 
tion as when asleep, but Brlicke has shown that this 
movement results from a different state of turgescence 
in the cells from that which occurs during sleep ; and as 
sleep-movements are certainly due to modified circumnu- 
tation, those from a touch can hardly be thus due. The 
back of a leaf of Drosera rotundifolia was cemented to 
the summit of a stick driven into the ground, so that it 
could not move in the least, and a tentacle was observed 
during many hours under the microscope ; but it exhib- 
ited no circumnutating movement, yet after being mo- 
mentarily touched with a bit of raw" meat, its basal part 
began to curve in 23 seconds. This curving movement 
therefore could not have resulted from modified circum- 
nutation. But when a small object, such as a fragment 
of a bristle, was placed on one side of the tip of a radi- 
cle, which we know is continually circumnutating, the 
induced curvature was so similar to the movement caused 
by geotropism, that we can hardly doubt that it is due 
to modified circumnutation. A flower of a Mahonia was 
cemented to a stick, and the stamens exhibited no signs 
of circumnutation under the microscope, yet when they 
were lightly touched they suddenly moved towards the pis- 
til. Lastly, the curling of the extremity of a tendril when 
touched seems to be independent of its revolving or cir- 
cumnutating movement. This is best shown by the part 
which is the most sensitive to contact, circumnutating 
much less than the lower parts, or apparently not at all.* 
Although in these cases we have no reason to believe 
that the movement depends on modified circumnutation, 
as with the several classes of movement described in this 
volume, yet the difference between the two sets of cases 
may not be so great as it at first appears. In the one set, 

* For tlie evidence on this Habits of Climbing Plants,' 1875, 
head, see the ' Movements and pp. 173, 174. 



Chap. XII. CONCLUDING REMARKS. 575 

an irritant causes an increase or diminution in the tur- 
gescence of the cells, which are already in a state of 
change ; whilst in the other set, the irritant first starts 
a similar change in their state of turgescence. Why a 
touch, slight pressure or any other irritant, such as elec- 
tricity, heat, or the absorption of animal matter, should 
modif}^ the turgescence of the affected cells in such a 
manner as to cause movement, we do not know. But a 
touch acts in this manner so often, and on such widely 
distinct plants, that the tendency seems to be a very gen- 
eral one ; and if beneficial, it might be increased to any 
extent. In other cases, a touch produces a very different 
effect, as with ]S"itella, in which the protoplasm may be 
seen to recede from the walls of the cell ; in Lactuca, in 
which a milky fluid exudes ; and in the tendrils of cer- 
tain Vitace^, Cucurbitacese, and Bignoniaceae, in which 
slight pressure causes a cellular outgrowth. 

Finally, it is impossible not to be struck with the re- 
semblance between the foregoing movements of plants 
and many of the actions performed unconsciously by 
the lower animals.* With plants an astonishingly small 
stimulus suflaces ; and even with allied plants one may be 
highly sensitive to the slightest continued pressure, and 
another highly sensitive to a slight momentary touch. 
The habit of moving at certain periods is itiherited both 
by plants and animals; and several other points of simili- 
tude have been specified. But the most striking resem- 
blance is the localisation of their sensitiveness, and the 
transmission of an influence from the excited part to an- 
other which consequently moves. Yet plants do not of 
course possess nerves or a central nervous system ; and we 

* Sachs remarks to nearly the Energ:ien ausgerustct sind, iihn- 

sanie effect : " Dass sich die le- licli.wiedio verschiedenen Siniu-s- 

bende Pflanzensubstanz derurt ncrvcii dcs Tliicn- " ( ' Arhciti'ii 

inuerlich differenzirt. dass ein- des B(.t. Inst, in \N urzbury, IJd. 

zehie Theile mit speeifiscduMi ii. 187<». p. 282). 



576 CONCLUDING REMARKS, Chap. XII. 

may infer that with animals such structures serve only for 
the more perfect transmission of impressions, and for the 
more complete intercommunication of the several parts. 
We believe that there is no structure in plants more 
wonderful, as far as its functions are concerned, than the 
tip of the radicle. If the tip be lightly pressed or burnt 
or cut, it transmits an influence to the upper adjoining 
- part, causing it to bend away from the affected side ; and, 
what is more surprising, the tip can distinguish between 
a slightly harder and softer object, by which it is simul- 
taneously pressed on opposite sides. If, however, the radi- 
cle is pressed by a simihir object a little above the tip, the 
pressed part does not transmit any influence to the more 
distant parts, but bends abruptly towards the object. If 
the tip perceives the air to be moister on one side than on 
the other, it likewise transmits an influence to the upper 
adjoining part, which bends towards the source of moist- 
ure. When the tip is excited by light (though in the case 
of radicles this was ascertained in only a single instance) 
the adjoining part bends from the light ; but when excited 
by gravitation the same part bends towards the centre of 
gravity. In almost every case we can clearly perceive the 
final purpose or advantage of the several movements. Two, 
or perhaps more, of the exciting causes often act simul- 
taneously on the tip, and one conquers the other, no doubt 
in accordance with, its importance for the life of the plant. 
The course pursued by the radicle in penetrating the 
., ground must be determined by the tip ; hence it has ac- 

I quired sucli diverse kinds of sensitiveness. It is hardly 

an exaggeration to say that the tip of the radicle thus 
endowed, and having the power of directing the move- 
ments of the adjoining parts, acts like the brain of one 
of the lower animals ; the brain being seated within the 
anterior end of the body, receiving impressions from the 
and directing the several movements. 



INDEX. 



A. 

Abies communis^ effect of killinpr or 
injuring the leading slioot, 187. 

pectinata^ effect of killing or 

injuring the leading slioot, 187. 

, affected hy^cidium tlatinum^ 

188. 

Ahronia umbellata^ its single de- 
veloped cotyledon, 79. 

, rudimentary cotyledon, 96. 

, rupture of the seed-coats, 106. 

Abutilo/i Darwi/iii, sleep of leaves 
and not of cotyledons, 318. 

, nocturnal movement of leaves, 

325. 

Acacia Farnesiana^ state of plant 
when awake and asleep, 387, 388. 

, appearance at night, 401. 

, nyctitropic movements of pin- 

nye, 408. 

-^ the axes of the ellipses, 410. 

lopkcmtha^ character of first 

leaf, 420. 

retinoides, circumnutation of 

young phyllode, 236. 

Acanthosicyos horrida^ nocturnal 
movement of cotyledon, 307. 

Acanthus camldahrum^ inequality 
in the two first leaves, 81. 

, petioles not arched, 5o8. 

latifolius, variability in first 

leaves, 81. 

mollis^ seedling, manner of 

breaking through ' the ground, 
SO. 

, circumnutation of young leaf, 

250, 270. 

sjjinosus^ 80. 

, movement of leaves, 250. 

Aden.aidherti pavonia^ nyctitropic 

inovements of leafiets, 379. 
jEeidiuvi elntiiinm, effect on the 

lateral branches of the silver fir, 

188, 



^'Esctdiis hippocastatinm^ move- 
ments of radicle, 28, 29. 

, sensitiveness of apex of radi- 
cle, 171-174. 

Albizzia lophantha^ nyctitropic 
movements of leafiets, 388. 

, of pinnaj, 408. 

AUium cepa, conical protuberance 
on arched cotyledon, GO. 

, circumnutation of basal half 

of arched cotyledon, 61. 

, mode of breaking through 

ground, 88. 

— — , straightening process, 102. 

pornun^ movements of tiower- 

stems, 225, 

Alopecurus pj'atensis^'^o'xnt^ affected 
by apogeotropism, 507. 

Aloysia citriodora^ circumnutation 
of stem, 209. 

Amaranthus^ sleep of leaves, 393. 

caudntus, nocturnal movement 

of cotyledons, 310. 

Amorpha fraticoaa^ sleep of leaflets, 
358. 

A?7ipelopsis tricnspidata, hyponns- 
tic movement of hooked tips, 
274-277. 

AmphicarpfBa monoica, circun)nu- 
tation and nyctitropic movements 
of leaves, 370. 

, effect of sunshine on Icafietf--, 

451. 

, geotropic movements of, 524, 

Anoila Wriijhtii^ sleep of cotyle- 
dons. 305, 315. 

, of leaves, 326. 

, downward movement of coty- 
ledons, 450. 

Apheliotropism, or negative lieliot- 
roi)ism, 5, 418, 438. 

Ap/'os (jravt'oh lift, hcliotropic move- 
ments of liypocotyl, 427-430. 

f'lbirottii^ vertical sinking of 

leallets at niglit, 372, 

577 



578 



INDEX. 



Apium graveolens^ sleep of cotyle- 
dons, b08. 

, petroselinum.^ sleep of cotyle- 
dons, 307. 

Apogeotropic movements effected 
by joints or pulvini, 506. 

Apogeotropism, 5, 498 ; retarded by 
heliotropisni, 506 ; concluding re- 
marks on, 512. 

Arachis hypogcea^ circuranutation of 
gynophore, 224. 

, ett'ects of radiation on leaves, 

2'Jl, 299. 

, movements of leaves, 360. 

, rate of movement, 410. 

, circumnutation of vertically 

dependent young gynophores, 
5::4. 

, downward movement of the 

same, 524. 

Arching of various organs, impor- 
tance of, to seedling plants, 88, 
89; emergence of hypocotyls or 
epicotyls in the form of an, 558. 

Asparagus officinalis^ circumnuta- 
tion of plunmles, 61-63. 

, effect of lateral light, 489. 

Asjylenium trichomanes^ movement 
in the fruiting fronds, 259, n. 

Astraaalus uliginosus^ movement of 
lealiets, 359. 

Avena sativa^ movement of cotyle- 
dons, 67. 

, sensitiveness of tip of radicle 

to moist air, 183. 

, heliotropic movement and cir- 
cumnutation of cotyledon, 426, 
427. 

, sensitiveness of cotyledon to a 

lateral light, 482. 

, young sheath-like cotyledons 

stronglyapogeotropic, 504. 

. movenients of oldish cotyle- 
dons, 505. 

Averrhoa biUmhi^ leaf asleep, 333. 

, angular movements when go- 
ing toX'^leep, 334-338. 

, leaflets exposed to bright sun- 
shine, 453. 

Azalea Indica, circumnutation of 
stem, 207, 208. 

B. 

Bary, de, on the effect of the iEci- 
dium on the silver fir, 188. 



BRASSICA. 

Batalin, Prof., on the nyctitropic 
movements of leaves, 286; on the 
sleep of leaves of Sida napcea^ 
324; on Polygonum nviculare^ 
393 ; on the ettect of sunshine on 
leaflets of Oxalis acetosella^ 453. 

Baukinia^ nyctitropic movements, 
378. 

, movements of petioles of young 

seedlings, 407. 

, appearance of young plants at 

night, 408. 

Beta vulgaris^ circumnutation of 
hypocotyl of seedlings, 53. 

^, movements of cotyledons, 53, 

54. 

, effect of light, 124. 

, nocturnal movement of coty- 
ledons, 310. 

, heliotropic movements of, 425. 

, transmitted ettect of light on 

hypocotyl, 487. 

, apogeotropic movement of hy- 
pocotyl, 50l. 

Bignoraa capreolata^ apheliotropic 
movement of tendrils, 438, 455. 

Bouclie on Melaleuca ericcefolia^ 
389. 

Brassica napus^ circumnutation of 
flower-stems, 225. 

Brassica oleracea, circumnutation of 
seedling, 9. 

, of radicle, 10. 

, geoti'opic movement of radicle, 

10. 

, movement of buried and 

arched hypocotyl, 12, 13, 14. 

— — , conjoint circuranutation of 
hypocotyl and cotyledons, 15, 16, 
17. 

— -, of hypocotyl in darkness, 18. 

i of a cotyledon with hypocotyl 

secured to a stick, 19. 

, rate of movement, 19. 

, ellipses described by hypo- 
cotyls when erect, 108. 

, movements of cotvledons, 115, 

, of stem, 201, "202. 

, of leaves at night, 229, 

230. 

, sleep of cotyledons, 303. 

, circumnutation of hypocotyl 

of seedling plant, 430. 

, heliotropic movement and 

circumnutation of hypocotyls, 
431. 



INDEX. 



579 



Brassica oleracea^ eft'ect of lateral 

light on hypocotyls, 484-487. 
, apogeotropic movement of 

hypocotyls, 505, 506. 
Brassica rapa, movements of leaves, 

230. 
Brongniart, A., on the sleep of 

IStrephi urn florib u ndum^ 397. 
Bruce, Dr., on the sleep of leaves in 

Averrhoa^ 333. 
Bryophyllani (vel Calaiichoe) calyci- 

lium^ movement of leaves, 237. 



C. 



Camellia Japoiiica^ circunmutation 
of leaf, 231, 232. 

Candolle, A. de, on Trapa nutans^ 
96 ; on sensitiveness of cotyle- 
dons, 128. 

Carina Warscewiczii, circunmuta- 
tion of plumules, 60. 

, of leaf, 253. 

Cannabis saliva^ movements of 
leaves, 251. 

, nocturnal movements of coty- 
ledons, 310, 

, sinking of the young leaves 

at night, 450. 

Cassia, nyctitropic movement of 
leaves, 374. 

Cassia Barclayana^ nocturnal move- 
ment of leaves, 377. 

, slight movement of leaflets, 

407. 

calliantha, uninjured by ex- 
posure at night, 292, n. 

, nyctitropic movement of 

leaves, 376. 
, circumnutating movement of 

leaves, 377. 

corymbosa^ cotyledons sensi- 
tive to contact, 127. 

, nyctitropic movement of 

loaves." 374. 

floribunda, use of sleep move- 
ments. 292. 

, effect of radiation on the 

leaves at night, 298. 

, circumnutating and nycti- 
tropic nioveuient of a terminal 
leafiet, 377, 378. 

, movements of young and older 

leaves, 407. 

Jlorida^ cotyledons sensitive to 

contact, 127. 



Cassia Barclayana^ sleep of cotyle- 
dons, 311. 

glauca^ cotyledons sensitive to 

contact, 127. 

, sleep of cotyledons. 311. 

Imvigata^ ettect of radiation on 

leaves, 292, n. 

raimosoides^ movement of coty- 
ledons, 117. 

, sensitiveness of, 127. 

, sleep of, 311. 

, nyctitropic movement of 

leaves, 377. 

, effect of bright sunshine on 

cotyledons, 446. 

— =— neqlecta, movements of, 117. 

, effect of light, 125. 

, sensitiveness of cotvledons, 

127. 

nodosa^ non-sensitive cotyle- 
dons, 127. 

, do not rise at night, 311. 

pubescens, non-sensitive coty- 
ledons, 127. 

,pi/bescens, uninjured by expo- 
sure at night, 296. 

, sleep of cotyledons, 311. 

, nyctitropic movement of 

leaves, 376. 

, circumnutating movement of 

leaves. 377. 

, nyctitropic movement of 

petioles, 406. 

, diameter of plant at night, 

408. 

sp. (?) movement of cotyledons, 

117. 

toi'a, circumnutation of coty- 
ledons and hypocotyls, 34, 35, 
110, 311. 

, ellbct of light, 125, 126. 

, sensitiveness to contact, 126. 

, heliotropic moveiiicnt and 

circunmutation of liypocotyl, 4;i6. 

, liypocotyl of seedling slightly 

heliotropic. 459. 

, apogeotrnpic movement of uld 

liypocotyl, 501 

'-, movement of hyi)Ocotyl of 

young seedling, 515. 

Cau.sticrnitrate of silver), effect of, 
on radicle of liean, 151, 15t'>; on 
tlie common pea, 161. 

Cells, tal)le ot the measurement 
of, in the ])ulvini o\' Uxalis 
coruicutata, 121 ; changes in, 552. 



580 



INDEX. 



CENTKOSEMA. 

Centrosema, 370. 

Ceratophyllum demersum^ move- 
ments of i>tem, 210, 

Cereus Landbeckii^ its rudimentary 
cotyledons, 98. 

speciossimus, circumnutation 

of stem, 205, 206. 

Cerinthe major^ circumnutation of 
hypocotyl, 50. 

^, of cotyledons, 50. 

, ellipses described by hypo- 

cotyls when erect, 108. 

eft'ect of darkness, 124. 

Chatin, M., on Pinus Nordman- 
niana^ 395. 

Chenopodium album, sleep of 
leaves, but not of cotyledons, 318, 
322. 

, movement of leaves, 393. 

Chlorophvll injured by bright light, 
452. 

Ciesielski, on the sensitiveness of 
the tip of the radicles, 4, 527. 

Circumnutation, meaning explained, 
1; modified, 265-282; and heliot- 
ropism, relation between, 441 ; of 
paramount importance to every 
plant, 555. 

Cissus discolor, circumnutation of 
leaf, 233. 

Citrus aurantium, circumnutation 
of epicotyl, 28. 

, unequal cotyledons, 95. 

Clianthus Dampieri, nocturnal 
movement of leaves, 300. 
Cob'.ea scandens, circumnutation 
of, 272. 

Cohn, on the water secreted by 
Lathrcea squamaria, 87, n. : on 
the movement of leaflets of Oxa- 
lis, 452. 

Colutea arborea, nocturnal move- 
ment of leaflets, 359. 

Couiferce, circumnutation of, 211. 

Coronilla rosea, leaflets asleep, 359. 

Corylus avellana, circumnutation of 
young shoot, emitted from the 
epicotyl, 56, 57. 

. arched epicotyl, 79. 

Cotyledon iimbilicns, circumnuta- 
tion of stolons, 219, 220. 

Cotyledons, rudimentary, 95-99 ; 
circumnutation of, 109-113; noc- 
turnal movements, 112, 113 ; pul- 
vini or joints of, 113-123; dis- 
turbed periodic movements by 



CYCLAMEN. 

light, 124; sensitiveness of, to 
contact, 126; nyctitropic move- 
ments of, 285, 3i0; list of coty- 
ledons which rise or sink 'at 
night, 303 ; concluding remarks 
on^their movements, 31 5. 

Crambe maritima, cii'cumnutation 
of leaves, 228, 229. 

Crinum capense, shape of leaves, 
255. 

, circumnutation of, 255. 

Crotolaria (sp. i), sleep of leaves, 
344. 

Cryptogams, circumnutation of, 258- 

261. 
Cucumis dudaim, movement of 
colyledons, 44. 

.'sleep of cotyledons, 307. 

Cucurbita aurantia, movement of 
hypocotyl, 42. 

"-, cotyledons vertical at night, 

307. 

ovifera, ereotropic movement 

of radicle, 39', 40. 

, circumnutation of arched hy- 
pocotyl, 40. 

, of straight and vertical hypo- 
cotyl, 41. 

, movements of cotyledons, 41, 

42, 115, 125. 

, position of radicle, 90. 

, rupture of the seed-coats, 103. 

, circumnutation of hypocot^'l 

when erect, 108. 

, sensitiveness of apex of radi- 
cle, 169-171. 

, cotyledons vertical at night, 

307. 

. not affected bv apogeotropism, 

514. 

, tips cauterised transversely, 

542. 

Curvature of the radir>le, 192. 

Cycas pectinata^ circumnutation of 
young leaf, whilst emerging from 
the STound, 59. 

, first leaf arched, 80. 

, circumnutation of terminal 

leaflets, 253. 

Cyclamen Persicum, movement of 
cotyledons, 47. 

,' undeveloped cotyledons, 79, 

80, 97. 

, circumnutation of peduncle, 

224. 

, , of leaf, 247, 248. 



INDEX. 



581 



CYCLAMEN. 

Cyclamen Persicum^ downward 
apheliotropic movemciiit of a 
flower peduncle, 439-441. 

, burying of the pods, 439. 

Cyperus alternifolius^ circuumuta- 
tion of stem, 211, 212. 

, movement of stem, 513 

Cytisus fragrans^ circumnutation of 
hypocotyl, 88. 

"-, sleep of leaves, 348, 403. 

. apogeotropic movement of 

stem, 496, 499. 



D. 



Dahlia^ circumnutation of young 
leaves, 245, 24(5. 

Dalea alopecuroides^ leaflets de- 
pressed at night, 359. 

Darkness, effect of, on the move- 
ment of leaves, 412. 

Darlingtonia Calif ornica^ its leaves 
or pitchers apheliotropic, 455, 7i. 

Darwin, Charles, on Maurandia 
semperflorens^ 225 ; on the Swed- 
ish turnip, 230, n. ; movements 
of climbing plants, 268, 273 ; the 
heliotropic movement of the ten- 
drils of Bigaonia caprcolata^ 438 ; 
revolution of climbing plants, 
456 ; on the curling of a tendril, 
574. 

, Erasmus, on the peduncles of 

Cyclamens, 439. 

, Francis, on the radicle of 

Sinapis alba^ 491 ; on Hygroscop- 
ic seeds, 494, n. 

Datura stramonium^ nocturnal 
movement of cotyledons, 301. 

Delpino, on cotyledons of Chiero- 
phyllum and Corydalis, 97, n. 

Delphinium nvdicavle^ mode of 
breaking through the ground, 81. 

. confl^uent petioles of two coty- 
ledons, 558. 

Desmodium gyrans, movement of 
leaflets, 259, n. 

, position of leaves at night, 

288. 

, sleep of leaves, not of coty- 
ledons, 318. 

, circumnutation and nycti- 

tropic movement of leaves, 362- 
367. 

• , movement of lateral leaflets, 

365. 



Desmodium gyrans^ jerking of leaf- 
lets, 366. 

, nyctitropic movement of peti- 
oles, 406, 407. 

, diameter of plant at night, 408. 

, lateral movement of leaves, 

410. 

, zigzag movement of apex of 

leaf, 411. 

, shape of lateral leaflet, 421. 

'»}es2)ertilionis, 368, n. 

Deutzia gracilis^ circumnutation of 
stem, 204. 

Diageotropism, 5 ; or transverse- 
geotropism, 525. 

Diaheliotropism, 5 ; or Transversal- 
lleliotropismus of Frank, 424; 
influenced by epinasty, 444; by 
weight and apogeotropism, 446. 

DUuithus caryopkijllus^ 230. 

, circumnutation of young leaf, 

231, 271. 

Dicotyledons, circumnutation wide- 
ly spread among, 70. 

Dionaja, oscillatory movements of 
leaves, 2(i2, 273. 

Dioticea muscipula, circumnutation 
of young expanding leaf, 240. 

, closure of the ^obes and cir- 
cumnutation of a full-grown leaf, 
241. 

, oscillations of, 242-245. 

Diurnal sleep, 424. 

Drosera Capensis^ structure of flrst- 
formed leaves. 420. 

rot n?idi folia, movement of 

young leaf, 238. 

, of the tentacles, 239. 

, sensitiveness of tentacles, 262. 

, shape of leaves, 420. 

, leaves not heliotropic, 455. 

, leaves circumnutate largely, 

459. 

, sensitiveness of, 574. 

Ducharlrc on TephroKia earibaa, 
359 ; on the nyctitropic movement 
of the Cassia, 374. 

Duval-Jouve, on the movements of 
Bryophyllum calyeiniim, 2!}7 ; <>f 
the narrow leaves of the Grami- 
neu', 419. 

Dyer, Mr. Thiselton, on the leaves 
of Crotolaria, 344 ; on Cassia tfur i- 
hiinda,'?,1A, n.\ on the iil>s(.Vbi-iit 
hairs on the hurit-d flowiT-licads 
of Tritnl i iini xnliti rriiin itiii. 522. 



582 



INDEX. 



ECHEVERIA. 



JEcheveria stolomfera^ circumnuta- 
tion of leaf, 287. 

Eckinocactus viridescens^ its rudi- 
mentary cotyledons, 98. 

Echinocystis lobata^ movements of 
tendrils, 268. 

, apogeotropism of tendrils, 515, 

Elfving, F., ou the rhizomes of 
Sparganium ramosnm^ 189; on 
the diageotropic movement in the 
rhizomes of some plants, 526. 

Elyiitus arenareus^ leaves closed 
during the day, 419. 

Embryology of leaves, 420. 

Engelmann, Dr., on the Quercus 
virens^ 86. 

Epinasty, 5, 269. 

Epicotyl, or plumule, 4 ; manner of 
breaking through the ground, 78 : 
emerges from the ground under 
the form of an arch, 558. 

Erytlirina caffra^ sleep of leaves, 
372. 

corallodendron^ movement of 

terminal leaflet, 372. 

Crista-galli^ effect of tempera- 
ture on sleep of leaves, 321. 

, circumnutation and nycti- 

tropic movement of terminal leaf- 
lets, 372. 

Eucalyptus resinifera^ circumnuta- 
tion of leaves, 245. 

Euphorbia jacquinicBJiora^ nycti- 
tropic movement of leaves, 394. 

F. 

Flahault, M., on the rupture of seed- 
coats, 102-106. 

Flower-stems, circumnutation of, 
222-226. 

Fragaria Rosacea^ circumnutation 
of stolon, 214-217. 

Frank, Dr. A. B., the terms Heliot- 
ropism and Geotropism, first used 
by him, 5, n. ; radicles acted on 
by geotropism, 72, n. ; on the sto- 
lons of Fragaria, 214; periodic 
and nyctitropic movements of 
leaves, 287 ; on the root-leaves of 
plants kept in darkness, 448 ; on 
pulvini. 490 ; ou natural selection 
in connection witli geotropism, 
heliotropism, &c., 573. 



GOSSYPIUM. 

Frank, Dr. A. B., on Transversal- 

Heliotropismus, 424. 
Fuchsia, circumnutation of stem, 

2U5. 

G. 

G-azania ringens, circumnutation of 
stem, 207. 

Genera containing sleeping plants, 
323, 324. 

Geotropism, 5 ; effect of, on the pri- 
mary radicle, 195; the reverse of 
apogeotropism, 517; effect on the 
tips of radicles, 548. 

Geraniuvi cinereum, 307. 

: Endressii, 307. 

Ibericum, nocturnal movement 

of cotyledons, 301. 

Bichardsoni, 307. 

rotundifoliuia, nocturnal 

movement of cotyledon, 307, 315. 

suhcaulescens, 307. 

Germinating seed, history of a, 553. 
Githago segetum, circumnutation of 

hypocotyl, 21, 108. 

, burying of hypocotyl, 109. 

, seedlings feebly illuminated, 

125, 129. 

, sleep of cotyledon, 305. 

, leaves, 323. 

Glauciu'm luteum, circumnutation 

of young leaves, 227, 228. 
Gleditschia, sleep of leaves, 373. 
Glycine hispida, vertical sinking of 

leaflets, 371. 
Glycyrrhiza, leaflets depressed at 

night, 359. 
Godlewski, Emil, on the turges- 

cence of the cells, 490. 
Gooseberry, effect of radiation, 287. 
Gossypium (var. Nankin cotton), 

circumnutation of hypocotyl, 22. 

, movement of cotyledon, 22, 23. 

, sleep of leaves, 327. 

arloreuin (?), sleep of cotyle- 
dons, 306. 

Braziliense, nocturnal move- 
ment of leaves, 327. 

herhaceum, sensitiveness of 

apex of radicle, 168. 

, sleep of cotyledons, 306. 

, radicles cauterised transverse- 

ly, 542. 

mantimum, nocturnal move- 
ment of leaves, 327. 



INDEX. 



583 



GEAVITATION. 

Gravitation, movements excited by, 
571. 

Gray, Asa, on DelpJiinium nudi- 
caule, 81 ; on Megarrhiza Calif or- 
nica^ 82 ; on the movements in 
the fruiting fronds of Asplenium 
trickomanes^ 259; on the AinpM- 
earpaia monolca^ 525 ; on the 
Ifomota Jalappa, 561. 

Grease, effect of, on radicles and 
their tips, 182, 185. 

Gressner, Dr. H., on the cotyledons 
of Cyclamen Persicum^ 46, 79 ; on 
hypocotyl of the same, 97. 

Gymnosperms, 395. 



Haberlandt, Dr., on the protuber- 
ance on the hypocotyl of Allium, 
60 ; the importance of the arch 
to seedling plants, 89 ; sub-aerial 
and subterranean cotyledons. 111, 
n. ; the arched hypocotyl, 559, 

Ho&maioxylon Campechianmn^ noc- 
turnal movement of leaves, 373, 
374. 

Hedera helix, circumnutation of 
stem, 206. 

Hedysarurn coronarium, nocturnal 
movement of leaves, 360. 

Heliantliemum prostratum, geotrop- 
ic movement of flower - heads, 
523. 

Helianthus annuus^ circumnutation 
of hypocotyl, 45. 

, arching of hypocotyl, 91. 

, nocturnal movement of coty- 
ledons, 308. 

Heliotropism, 5 ; uses of, 454 ; a 
modified form of circumnutation, 
495. 

Hellehorus niger^ mode of breaking 
through the ground, 87. 

Hensen, Prof., on roots in worm- 
burrows, 74. 

Henslow, Kev. G., on the cotyle- 
dons of Phalaris Camiriensis, 63. 

Hofmeister, on the curious move- 
ment of Spirogyra, 3, 260, n. ; of 
the leaves of Pistia stratiotes, 
256 ; of cotyledons al night, 300 ; 
of petals, 419. 

and Batalin on the movements 

of the cabbage, 229. 

Hooker, Sir J., on the effect of light 

38 



on the pitchers of Sa)'racenia, 
455. 

Hypocotyl, 4 ; manner of breaking 
through the ground, 78 ; emerges 
under the form of an arch, 558. 

Hypocotyls and Epicotyis, circum- 
nutation and other movements 
when arched, 99 ; power of 
straightening themselves, 101 ; 
rupture of the seed-coats, 102- 
107 ; illustration of, 107 ; circum- 
nutation when erect, 107 ; when 
in dark, 109. 

Hyponasty, 5, 269. 



Iberis umbellata^ movement of stem, 
201. 

Illumination, effect of, on the sleep 
of leaves, 404. 

Imatophyllum vel Clivia (sp. ?), 
movement of leaves, 256. 

Indigqfera tinctoria^ leaflets de- 
pressed at night, 359. 

Inheritance in plants, 413, 490. 

Insectivorous and climbing plants 
not heliotropic, 455 ; influence of 
light on, 493. 

IpomoAa hona-nox^ arching of hypo- 
cotyl, 91. 

, nocturnal position of cotyle- 
dons, 309, 316. 

ccerulea vel Pharbitis nil, cir- 
cumnutation of seedlings, 47. 

, movement of cotyledons, 48- 

50, 110. 

, nocturnal movement of coty- 
ledons, 308. 

, sleep of loaves, 392. 

, sensitiveness to light, 456. 

, the hypoootylcdonous stems 

heliotropic, 458. 

, coccinea, position of cotyle- 
dons at night, 309, 316. 

Uptopkylla, mode of breaking 

through the ground, 85, 86. 

, arching of the petioles of tlio 

cotyledons, 91. 

, difference in sensitiveness to 

gravitation in different parts. 514. 

, extraordinary manner of ger- 
mination, 561. 

pandurata, manner of germ- 
ination. 86, 561. 

purpurea (vel Pharbitis his- 



584 



INDEX. 



nocturnal movement of 
cotyledons, 308, 316. 

Ijpomo&a purpurea^ sleep of leaves, 
392. 

, sensitiveness to light, 456. 

, the hypocotyledonous stems 

heliotropic, 458. 

Iris pseudo-acorus^ circumnutation 
of leaves, 254. 

Irmisch, on cotyledons of Ranun- 
culus Ficaria^ 97. 

Ivy, its stems heliotropic, 456. 



Kerner on the bending down of pe- 
duncles, 419. 

Klinostat, the, an instrument de- 
vised by Sachs to eliminate geot- 
ropism, 94. 

Kraus, Dr. Carl, on the underground 
shoots of Triticum repens, 189 ; 
on Cannabis sativa^ 251, 310, 315 ; 
on the movements of leaves, 321. 

L. 

Lactuca scariola, sleep of cotyle- 
dons, 308. 

Lagenaria vulgaris^ circumnutation 
of seedlings, 43. 

, of cotyledons, 44. 

, cotyledons vertical at night, 

307. 

Lathrcea squamaria, mode of break- 
ing through the ground, 87. 

, quantity of water secreted, 

87, w. 

Lathyrus nissolia^ circumnutation 
of stem of young seedling, 33. 

, ellipses described by, 108. 

Leaves, circumnutation of, 226- 
264 : dicotyledons, 226-253 ; mo- 
nocotyledons, 253-258 ; nyctitro- 
pism of, 283 ; their temperature 
affected by their position at night, 
297 ; nyctitropic or sleep move- 
ments, 318, 400 ; periodicity of 
their movements inherited, 412 ; 
embryology of, 420 ; so-called 
diurnal sleep, 451. 

Leguminosoi, sleep of cotyledons, 
311 ; sleeping species, 343. 

Le Maout and Decaisne, 09. 

Lepidium sativum^ sleep of cotyle- 
dons, 304. 



Light, movements excited by, 423, 
567 ; influence on most vegetable 
tissues, 491 ; acts on plants as on 
the nervous system of animals, 492. 

Lilium rtwra^c^m, circumnutation of 
stem, 211. 

', apogeotropic movement of 

stem, 503. 

Linnaeus, ' Somnus Plantarum,' 283 ; 
on plants sleeping, 323 ; on the 
leaves of ISida ahutiioit^ 326 ; on 
(Enothera moUissima, 389. 

Linum Berendieri, nocturnal move- 
ment of cotyledons, 301. 

usitatissimum, circumnutation 

of stem, 202. 

Lolium perenne., joints affected by 
apogeotropism, 507. 

Lonicera hrachypoda^ hooking of 
the tip, 274. 

, sensitiveness to light, 457. 

Loomis, Mr., on the movements in 
the fruiting fronds of Asplenium 
trichomanes, 259. 

Lotus aristata^ effect of radiation on 
leaves, 295. 

Creticxis^ leaves awake and 

asleep, 358. 

Gehelii^ nocturnal movement 

of cotyledons, 312. 

, leaflets provided with pulvini, 

357. 

JacolcBus^ movements of coty- 
ledons, 110. 

, pulvini of, 115. 

, movements at night, 117, 122, 

125. ■ 

, development of pulvini, 123 

, sleep of cotyledons, 312, 316 

, nyctitropic movement 

leaves, 357. 

major^ sleep of leaves, 357. 

perigrinus, movement of leaf- 
lets, 357. 

Lunvlaria vulgaris^ circumnutation 
of fronds, 259. 

Lupinvs, 344. 

alhifrons, sleep of leaves, 347. 

Hartwegii, sleep of leaves. 345. 

• luteus, circunmutation of coty- 
ledons, 38, 111. 

, effect of darkness, 124. 

, position of leaves when asleep, 

344. 

, different positions of leaves at 

night, 345. 



of 



INDEX. 



585 



Lupinvs^ varied movements of 

leaves and leaflets, 401. 

Menziesii^ sleep of leaves, 347. 

mutabilis^ sleep of leaves, 347. 

nanus^ sleep of leaves, 347. 

pilosus^ sleep of leaves, 344. 

polyphyllus, sleep of leaves, 

347. 
puhescens^ sleep of leaves by 

day and nigiit, 345, 346. 
, position of petioles at night, 

346. 

, movements of petioles, 407. 

speciosus^ circumnutation of 

leaves, 236. 
Lynch, Mr. E., on Pachira aqua- 

tica^ 96, n. ; sleep movements of 

Averrhoa. 333. 



Maranta arundinacea^ nyetitropic 
movement of leaves, 396, 397. 

, after much agitation do not 

sleep, 322. 

Marsilea quadrifoliata^ eftect of ra- 
diation at niglit, 295. 

, circunmutation and nyeti- 
tropic movement of leaflets, 398- 
400. 

, rate of movement, 410. 

Martins, on radiation at night, 287, n. 

Masters, Dr. Maxwell, on the lead- 
ing shoots of the Coniferce,2\\. 

Manrandia semperflorens. circum- 
nutation of peduncle, 224. 

Medicago maculata^ nocturnal posi- 
tion of leaves, 349. 

marina^ leaves awake and 

asleep, 348. 

Meehan, Mr., on the effect of an 
^cidium on Portulaca oleracea, 
189. 

MegarrJiiza Californica, mode of 
breaking through the ground, 82. 

, germination described by Asa 

Gray, 83, 84. 

, singular manner of germina- 
tion, 84, 561. 

Melaleuca ericoefolia^ sleep of leaves, 
389. 

Melilotus., sleep of leaves, 340. 

alba^ sleej) of leaves, 351. 

cmrulea, sleep of leaves, 351. 

dentata^ ettect of radiation at 

night, 299, 



Melilotus elegans, sleep of leaves, 

351. 

gracilis^ sleep of leaves, 351. 

infesta^ sleep of leaves, 351. 

Italica^ leaves exposed at night, 

294. 

, sleep of leaves, 351. 

macrorrkiza^ leaves exposed at 

night, 294. 

, sleep of leaves, 351. 

, messanensis, sleep of leaves on 

full-grown and voung plants, 352, 

421. 
officinalis, effect of exposure of 

leaves at night. 293, 299. 
, nocturnal movement of leaves, 

349, 351. 

, circumnutation of leaves, 352. 

, movement of petioles, 407. 

Melilotus parviflora, sleep of leaves, 

351. 
pttitpierreana, leaves exposed 

at night, 294, 299. 
, sleep of leaves, 351. 

secundijiora^ sleep of leaves, 

351. 

sunveolens, leaves exposed at 

night, 294. 

, sleep of leaves, 351. 

sulcata, sleep of leaves, 351. 

Taurica, leaves exposed at 

night, 294. 

, sleep of leaves, 351, 421. 

Methods of observation, 5. 
Mimosa alhida, cotyledons vertical 

at night, 117. 

, not sensitive to contact, 128. 

, sleep of cotyledons, 311. 

, rudimentary leaflets, 369. 

, nyetitropic movements of 

leaves, 385. 
, circumnutation of the main 

petiole of young leaf, 387. 
, torsion, or rotation of leaves 

and leaflets, 406. 

, first true leaf, 421. 

, eflect of bright sunshine on 

basal leaflets, 451. 

marginata, nyetitropic move- 
ments of leaflets, 387. 

jiudica, movement of coty- 
ledons, 105. 

, rupture of the seed-coats, 105. 

, circumnutation of cotyledons, 

110. 
, pulvini of, 114, 115. 



586 



INDEX. 



Mimosa pudica, cotyledons vertical 

at night, 116. 
, liardly sensitive to contact, 

128. 
, effect of exposure at night, 

296. 
, nocturnal movement of leaves, 

800. 

, sleep of cotyledons, 311. 

, . circumnutation and nycti- 

tropic movement of main petiole, 

379-384 

, of leaflets, 384. 

, circumnutation and nyctitropic 

movement of pinnae, 408. 
, number -of ellipses described 

in given time, 411. 
, effect of bright sunshine on 

leaflets, 452. 
Mirabilis jalapa and longiflora, 

nocturnal movements of cotyle- 
dons, 310. 
, nyctitropic movement of 

leaves, 393. 
Mohl, on heliotropism in tendrils, 

stems, and twining plants, 455. 
Momeutum-like movement, the ac- 
cumulated effects of apogeotro- 

pism, 513. 
Monocotyledons, sleep of leaves, 

395. 
Monotropa Jiypopitys^ mode of 

breaking through the ground, 

87. 
Morren, on the movements of 

stamens of Sparmannia and 

Cereus, 226. 
Miiller, Fritz, on Cassia tora^ 34; 

on the circumnutation of Linum 

usitatissimum^ 202 ; movements 

of the flower-stems of an Alisma, 

225.^ 
Mutisia clematis^ movement of 

leaves, 247. 
, leaves not heliotropic, 456. 

N. 

Natural selection in connection 

with geotropism, heliotropism, 

&c., 573. 
Nephrodium molle^ circumnutation 

of very young frond, 67. 

, of older frond, 258. 

, slight movement of fronds, 

514. 



I\eptunia oleracea^ sensitiveness to 
contact, 129. 

, nyctitropic movement of leaf- 
lets, 379. 

, of pinnae, 408. 

Mcotiaiia glauca^ sleep of leaves, 
391, 392. 

, circumnutation of leaves, 392. 

Nobbe, on the rupture of the seed- 
coats in a seedling of Marty nia^ 
106. 

iVoZawa^ros^ra^a, movement of seed- 
lings in the dark, 50. 

, circumnutation of seedling, 

109. 

Nyctitropic movement of leaves, 
565. 

Nyctitropism, or sleep of leaves, 
284 ; in connection with radia- 
tion, 289 ; object gained by it, 
418. 

0. 

Observation, methods of, 5. 

(Enotkera moUissima, sleep of 
leaves, 389. 

Opuntia basilaris^ conjoint circum- 
nutation of hypocotyl and cotyle- 
don, 45. 

, thickening of the hypocotyl, 

97. 

, circumnutation of hypocotyl 

when erect, 108. 

, burying of, 109. 

Orange, seedling, circumnutation 
of, 515. 

Orchis pyramidnliSs complex move- 
ment of pollinia, 494. 

Oxalis aeetosella, circumnutation of 
flower-stem, 223. 

, effects of exposure to radia- 
tion at night, 290, 291, 299. 

, circumnutation and nycti- 
tropic movement in full-grown 
leaf, 329. 

, circumnutation of leaflet when 

asleep, 329. 

, rate ot circumnutation of leaf- 
lets, 410. 

, effect of sunshine on leaflets, 

453. 

, circumnutation of peduncle, 

511. 

, seed-capsules, only occasion- 
ally buried, 523. 



INDEX. 



587 



Oxalis articulata^ nocturnal move- 
ments of cotyledons, 311. 

{Biophytum) sensitiva. rapid- 
ity of movement of cotyledons 
during the day, 26. 

, pulvinus of, 114. 

, cotyledons vertical at night, 

116, 118. 

bupleurifblia^ circumnutation 

of foliaceou's petiole, 331. 

, nyctitropic movement of ter- 
minal leaflet, 332. 

carnosa^ circumnutation of 

flower-stem, 222. 

, epinastic movements of flower- 
stem, 508. 

, effect of exposure at night, 290, 

299. 

, movements of the flower-pe- 
duncles due to apogeotropism and 
other forces, 508-511. 

corniculata (var. cuprea)^ 

movements of cotyledons, 26. 

, rising of cotyledons, 116. 

, rudimentary pulvini of cotyle- 
dons, 120. 

, development of pulvinus, 123. 

, effect of dull li^ht, 125. 

. experiments on leaves at night, 

291. 

florihunda^ pulvinus of cotyle- 
dons, 114. 

, nocturnal movement, 118, 311, 

316. 

fragrans, sleep of leaves, 327. 

Oi'tegesii^ circumnutation of 

flower-stems, 224. 

, sleep of large leaves, 330. 

, diameter of plant at night, 

408. 

, large leaflets affected by bright 

sunshine, 453. 

Flumierii^ aleep of leaves, 330. 

purpurea^ exposure of leaflets 

at night, 297. 

rosea, circumnutation of cotyle- 
dons, 23, 24. 

, pulvinus of, 113. 

, movement of cotyledons at 

night, 118, 311. 

, effect of dull light, 125. 

, non-sensitive cotyledons, 129. 

aenaitiva, movement of cotyle- 
dons, 110, 128, 129. 

, circumnutation of flower-stem, 

224. 



Oxalis sensitiva^ nocturnal move- 
ment of cotyledons, 311, 315. 
, sleep of leaves, 330. 

tropaoloides^ movement of 

cotyledons at night, 119, 120. 

Faldiviana, conjoint circum- 
nutation of cotyledons and hypo- 
cotyl, 24. 

, cotyledons rising vertically at 

night, 114, 116, 118. 

, non - sensitive cotyledons, 

129. 

, nocturnal movement of cotyle- 
don, 311, 315. 

, sleep of leaves and not of coty- 
ledons, 318. 

, movements of leaves, 329. 



Pachira aquatica^ unequal cotyle- 
dons, 96, n. 

Pancratium littorale^ movement of 
leaves, 256. 

Paraheliotropism, or diurnal sleep 
of leaves, 450. 

Passijiora gracilis^ 
and nyctitropic 
leaves, 389, 390. 

, apogeotropic 

tendrils, 515. 

, sensitiveness of tendrils, 555. 

Pelargo7numzonale^Q.\rQ\xmn\xiix{.ion 
of stem, 202. 

, and downward movement of 

young leaf, 232, 233, 271. 

Petioles, the, rising of, beneficial to 
plant at night, 408. 

Petunia violacea^ downward move- 
ment and circumnutation of very 
young leaf, 249, 271. 

Pfeft'er, Prof., on the turgescence of 
the cells, 2 ; on pulvini of leaves, 
114, 117 ; sleep movements of 
leaves, 283, 286 ; nocturnal rising 
of leaves of Malva, 326; move- 
ments of leaflets in Dcsnwdimn, 
gj/rans, S(i2; on Phi/ndiithns Xi- 
ruri, 395 ; influence of a pulvinus 
on leaves. 403 ; periddic move- 
ments of sleeping leaves, 412, 
413; movements of petals, 419; 
effect of britrlit sunshine on leaf- 
lets of Kobiniu, 451 : effect of 
light on parts provided witli pul- 
vini, 365. 



circumnutation 
movement of 

movement of 



588 



INDEX. 



PHALARIS. 

Phalaris Cavariensis^ movements of 
old seedlings, 63. 

, circumnutation of cotyledons, 

64, 108. 

, heliotropic movement and cir- 
cumnutation of cotyledon towards 
a dim lateral light, 432, 433. 

, sensitiveness of cotyledon to 

light, 459. 

, effect of exclusion of light 

from tips of cotyledons, 461. 

, manner of bending towards 

light, 461. 

, effects of painting with Indian 

ink, 472. 

, transmitted effects of light, 

473. 

, lateral illumination of tip, 474. 

, apogeotropic movement of the 

sheath-like cotyledons, 500. 

, change from a straight up- 
ward apogeotropic course to cir- 
cumnutation, 603. 

, apogeotropic movement of 

cotyledons, 504. 

Phaseolus Hernandesii^ nocturnal 
movement of leaves and leaflets, 
373. 

caracalla^ 94. 

, nocturnal movement of leaves, 

373. 

, effect of bright sunshine on 

leaflets, 451. 

multifioriis^ movement of radi- 
cles, 29. 

, of young radicle, 74. 

, of hypoeotyl, 93, 94. 

, sensitiveness of apex of radi- 
cle, 164-167. 

, to moist air, 181. 

, cauterisation and grease on 

the tips, 541. 

Roxhurghii^ effect of bright 

sunshine on first leaves, 451. 

, nocturnal movement of leaves, 

373. 

, nycti tropic movement of the 

first unifoliate leaves, 403. 

, vulgaris, 94. 

, sleep of leaves, 321. 

, vertical sinking of leaflets at 

night, 373. 

Fhyllanthus Nirxiri, sleep of leaf- 
lets, 395. 

Filoeerevs Houlletii^ rudimentary 
cotyledons, 98. 



PRINGSHEIM. 

Fimelia linoides, sleep of leaves, 
394. 

spectabilis^ sleep of leaves, 

394. 

Pincers, wooden, through which 
the radicle of a bean was allowed 
to grow, 77. 

Pinus auHtriaca^ circumnutation of 
leaves, 253. 

JSordmanniana^ nyctitropic 

movement of leaves, 395. 

2)inastei\ circumnutation of 

hypoeotyl, 57. 

, movement of two opposite 

cotyledons, 58. 
, circumnutation of young leaf, 

251, 252. 
, epinastic do\vnward move- 
ment of young leaf, 271, 272. 
Pistia stratiotes, movement of 

leaves, 256. 
Puum mtivum^ sensitiveness of 

apex of radicle, 158, 159. 
, tips of radicles cauterised 

transversely, 539. 
Plants, sensitiveness to light, 454 ; 

hygroscopic movements of, 494. 
Plants, climbing, circumnutation of, 

266; movements of, 563. 
, mature, circumnutation of, 200- 

213. 
Pliny on the sleep-movements of 

plants, 283. 
Plumbago Capensis^ circumnutation 

of stem, 207, 208. 
Poinciana Gitliesii, sleep of leaves, 

373. 
Polygonum amciilare^ leaves verti- 
cal at night, 393. 

convolvulus^ sinking of the 

leaves at night, 321. 

Povtederia (sp. ^) circumnutation 
of leaves, 257. 

Porlieria Tiygrometrica^ circumnu- 
tation and nvctitropic movements 
of petiole of "leaf, 338, 339. 

, eflect of watering, 339-341. 

, leaflets closed during the day, 

419. 

Portulaca oleracea^ effect of -^ci- 
dium on, 189. 

Primula Sinensis, conjoint circum- 
nutation of hypoeotyl and coty- 
ledon, 46. 

Prinafsheim on the injury to chloro- 
phyll, 452. 



INDEX. 



589 



PROSOPIS. 

Frosopis, nyctitropic movements of 
leaflets, 379. 

Psoralea acaulis, nocturnal move- 
ments of leaflets, 358. 

Pteris aquiiina^ rachis of, 88. 

Fulvini, or joints ; of cotyledons, 
113-123; influence of, on the 
movements of cotyledons, 317 ; 
effect on nyctitropic movements, 
402. 



Q. 



Q,uercus (American sp.), circumnu- 
tation of young stem, 54, 55. 

robur, movement of radicles, 

55, 56. 

, sensitiveness of apex of radi- 
cle, 174-176. 

Quei'cus virens^ manner of germina- 
tion, 86, 561. 



B. 

Radiation at night, effect of, on 
leaves, 287-289. 

Eadicles, manner in v/hich they 
penetrate the ground, 71-78 ; cir- 
cumnutation of, 71 ; experiments 
with split sticks, 76 ; with wood- 
en pincers, 77 ; sensitiveness of 
apex to contact and other irri- 
tants, 130; of Vicia faha, 133- 
158 ; various experiments, 136- 
142; summary of results, 144-152 ; 
power of an irritant on, com- 
pared with geotropism, 152-154; 
sensitiveness of tip to moist air, 
180; with greased tips, 185; effect 
of killing or injuring the primary 
radicle, 186-190; curvature of, 
192; affected by moisture, 198; 
tip alone sensitive to geotropism, 
545; protrusion and circuinnuta- 
tion in a germinating seed. 553 ; 
tip highly sensitive, 555 ; the tip 
acts like the brain of one of the 
lower animals, 576. 

, secondary, sensitiveness of the 

tips in the bean, 154; become 
vertically geotropic, 186-190. 

Ramey on the movements of the 
cotyledons of Mimosfi pndica^ 
and Cliardhus Dampieri at night, 
300. 



Jianu7iculus Ficaria^ mode of 
breaking through the ground, 
87, 91. 

, single cotvlcdon, 97. 

, effect of lateral light, 490. 

Raphauus sativus^ sensitiveness of 
apex of radicle, 171. 

, sleep of cotyledons, 3i 14. 

Rattan, Mr., on the germination of 
the seeds of Megarrhiza Calif or- 
iiica, 84. 

Relation between circumnutation 
and heliotropism, 441. 

Reseda odoruta, hypocotyl of seed- 
ling slightly heliotropio, 459. 

Reversion, due to nmtilation, 189. 

Ehipsalis cassytha^ rudimentary 
cotyledons, 98. 

Biciiius Borhonlensis, circumnuta- 
tion of arched hypocotyl, 54. 

Rohiiiia, effect of bright sunshine 
on its leaves, 451. 

pseudo-acacia^ leaflets vertical 

at night, 359. 

Rodier,' M., on the movements of 
Ctratophyllum demersum, 210. 

Royer, Ch., on the sleep-movements 
of plants, 284, u. ; on the sleep of 
leaves, 321 ; the leaves of Medi- 
cago maculata^ 348 ; on Wistaria 
Sinensis^ 359. 

Rubus idoius (hybrid) circumnuta- 
tion of stem, 204. 

, apogeotropic movement of 

stem, 502. 

Ruiz and Pavon, on Forlieria hy- 
gromett'ica^ 339. 

S. 

Sachs on "revolving nutation," 1; 
intimate connection between tur- 
gescence and growth, 2, n. ; coty- 
ledon of the onion, 60; adapta- 
tion of root-hairs, 71 ; the move- 
ment of the radicle, 72, 74, 75 ; 
movement in the hyi)ocotyls of 
the bean, &c., 92; sensitivent'ss 
of radicles, 132, 146, 197; sensi- 
tiveness of tiie primary radicle 
in the bean, 156; in the common 
pea, 157; etlW-t of moist air, ISO; 
of killing or injurinir tiic primary 
radicle, 186, 187; circumnutation 
of flower-stems, 225; cpiiuisty, 
269 ; movements of hatUts of 



590 



INDEX. 



SARRACENIA. 

Trifolium incdrnatum^ 354; ac- 
tion of light in modifying the 
periodic movements of leaves, 
423 ; on geotropisni and heliotro- 
pism, 442, n. ; on Tropaolum 
majus^ 458 ; on the hypocotyls 
slightly helioti'opic, and stems 
strongly apheliotropic of the ivy, 
458; heliotropism of radicles, 
488 ; experiments on tips of radi- 
cles of bean, 527, 528 ; curvature 
of the hypocotyl, 559; resem- 
blance between plants and ani- 
mals, 575. 

Sca^racenia purpurea^ circumnuta- 
tion of young pitcher, 227. 

Sazifraga sarnientosa^ circumnuta- 
tion of an inclined stolon, 217, 
218. 

Schraiikia aculeata^ nyctitropic 
movement of the pinnae, 387, 
408. 

uncinata, nyctotropic move- 
ments of leaflets, 387. 

Securigera coronilla^ nocturnal 
movements of leaflets, 357. 

Seed-capsules, burying of, 518. 

Seed-coats, rupture of, 102-107. 

Seedling plants, circumnutating 
movements of, 9. 

Selaginella^ circumnutation of, 259. 

lirayssii (?), circunmutation of 

young plant, 68. 

Sida napcea. depression of leaves at 
night, 324. 

, no palvinus, 324. 

retusa^ vertical rising of leaves, 

324. 

rJiomhifolia^ sleep of cotyle- 
dons. 312. 

, sleep of leaves, 318. 

, vertical rising of leaves, 324. 

, no pulvinus, 324. 

, circumnutation and nycti- 
tropic movements of leaf of young 
plant, 324. 

\ nvctitropic movement of 

leaves," 403. 

Siegesbeckia orientalis^ sleep of 
leaves, 322, 390. 

ISiiiapis alba^ hypocotyl bending to- 
wards the light, 466. 

, transmitted etfect of light on 

radicles, 487, 488, 571. 

, growth of radicles in dark- 
ness, 491. 



Sinapis nigra, sleep of cotyledons, 
304. 

Smilax aspera, tendrils aphelio- 
tropic, 455. 

Srnitkia Pfundii, non - sensitive 
cotyledons, 128. 

, hyponastic movement of the 

curved summit of the stem, 277- 
279. 

, cotyledons not sleeping at 

night, 311. 

vertical movement of leaves, 

360. 

sensitiva, sensitiveness of coty- 
ledons to contact, 128. 

, sleep of cotyledons, 311. 

iSophora chrysophyUa, leaflets rise 
at night, 373. 

Solanum dulcamara, circumnutat- 
ing stems, 268. 

lycopersicum, movement of hy- 
pocotyl, 51. 

, of cotyledons, 51. 

, ett'ect of darkness, 124. 

, rising of cotyledons at night, 

310. 

, heliotropic movements of hy- 
pocotyl, 426. 

, effect of an intermittent light, 

462. 

, rapid heliotropism, 466. 

palinacantTium, circumnuta- 
tion of arched hvpocotvl, 52, 
101. 

, of cotyledon, 52. 

, ellipses described by hypo- 
cotyl when erect, 108. 

— ^, nocturnal movement of coty- 
ledons, 310. 

Sparganium ramos^im, rhizomes of, 
189. 

SphoiropJiysa salsola, rising of leaf- 
lets, 359. 

Spirogyra princeps, movements of, 
260, 71. 

Stahl, Dr., on the ett'ect of ^ci- 
dium on shoot, 189; on the influ- 
ence of light on swarm-spores, 
493, n. 

Stapelia sarpedon, circumnutation 
of hypocotyl, 47. 

, minute' cotyledons, 98. 

iStdlaria medial nocturnal move- 
ment of leaves, 300. 

Steins, circumnutation of, 201-213. 

Stolons, or Runners, circumnuta- 
tion of, 213-222, 563. 



INDEX. 



591 



STRASBURGER. 

Strasburger, on tlie effect of light 
on spores of Hgeinatoceus, 4G0, a. ; 
the influence of light on tlie 
swarm- spores, 493. 

Strawberry, stolons of the, circum- 
nutate, but not affected by mod- 
erate light, 459. 

Strephium Jioribundum^ circumnu- 
tation and nyctitropic movement 
of leaves, 397, 398. 



Tamarindus Indica^ nyctitropic 
movement of leaflets, 379. 

Transversal - heliotropismus (of 
Frank) or diaheliotropism, 444. 

Trapa natans^ unequal cotyledons, 
96, n. 

Tecoma^ radicans^ stems aphelio- 
tropic, 456. 

Tephrosia caribcBa^ 359. 

Terminology, 4. 

Thalia dealbata^ sleep of leaves, 
395. 

, lateral movement of leaves, 

410. 

Trichosanthes ariguina^ action of the 
peg on the radicle, 105. 

nocturnal movement of coty- 
ledons, 307. 

7Vi/b/*vm, position of terminal leaf- 
lets at night, 285. 

glohosv.ni^ v/ith hairs protect- 
ing the seed-bearing flowers, 522. 

glomeratum, movement of coty- 
ledons, 312. 

incarnatum^ movement of coty- 
ledons, 312. 

Pannonic'um. shape of first 

true leaf, 354, 420. 

pratense, leaves exposed at 

night, 297. 

■ repens^ cii'cumnutation of flow- 
er-stem, 224. 

, circumnutating and epinastic 

movements of flower-stem, 279- 
281. 

, nyctitropic movement of 

leaves, 353. 

, circumnutation and nycti- 
tropic movements of terniinal 
leaflets, 356, 357. 

, sleep movements, 353. 

resvpi7iatum, no pulvini to 

cotyledons, 118. 



TROP.EOLUM. 

Trifolium resupinatum^ circumnu- 
tation of stem, 203. 

, eflect of exposure at night, 

298. 

, cotyledons not rising at night, 

118, 312. 
, circunmutation and nycti- 
tropic movements of terminal 
leaflets, 355, 356. 
strictum^ movements of coty- 
ledons at night, 117, 118. 
, nocturnal and diurnal move- 
ments of cotyledons, 313, 314, 
317. 

, movement of the left-hand 

cotyledon, 319. 

sxihtirrane'wm,^ movement of 

flower-heads, 73. 

, of cotyledons at night, 117, 

118, 312. 

, circumnutation of flower-stem, 

224, 225. 
, circumnutation and nycti- 
tropic movements of leaves, 354. 

, number of ellipses in 24 

hours, 411. 

, burying its flower-heads, 518. 

, downward movement of pe- 
duncle, 518. 

, circumnutating movement of 

peduncle, 519. 
Trigonella Cretica^ sleep of leaves, 

348. 
Triticum repens, underground 
shoots of, become apogeotropic, 
189. 
Tr-iticum vulgare^ sensitiveness of 

tips of radicle to moist air. 184. 
Tropceolum majvs ('^), sensitiveness 
of apex of radicle to contact, 
167. 

, circumnutation of stem, 203. 

, influence of illumination on 

nyctitropic movements, 341-343, 
347. 
, heliotropic movement and cir- 
cumnutation of epicotyl of a young 
seedling, 434. 

, of an old internode towards a 

lateral light, 436. 

, stems of very young plants 

highly heliotropic, of old plants 
slightlv apheliotropic, 458. 

, eflect of lateral light, 490. 

minus (?), circumnutation of 

buried and arched epicotyl, 27. 



592 



INDEX. 



Ulex, or gorse, first-formed leaf of, 

420. 
Urarii lagopus, vertical sinking of 

leaflets at night, 370. 

V. 

Vaucher, on the burying of the 
flower-heads of Trifolium sub- 
terraneum^ 518 ; on tke protection 
of seeds, 522. 

Verbena melindres (?), circuranuta- 
tion of stem, 209. 

, apogeoti'opic movement of 

stem, 499. 

Vicia faba^ circumnutation of radi- 
cle, 29, 30. 

, of epicotyl, 31-33. 

, curvature of hypocotyl, 93. 

, sensitiveness of apex of radicle, 

133-135. 

, of the tips of secondary radi- 
cles, 154. 

, of the primary radicle above 

the apex, 155-159. 

, various experiments, 136-144. 

, summary of results, 144-152. 

, power of an irritant on, com- 
pared with that of geotropism, 
152-154. 
Vicia faba^ circumnutation of leaves, 
233-236. 

, circumnutation of terminal 

leaflet, 235, 236. 

, eff'ect of apogeotropism, 449. 

, efl'ect of amputating the tips 

of radicles, 527. 

, regeneration of tips, 530. 

, snort exposure to geotropic 

action, 531. 

, eftects of amputating the tips 

obliquely, 533. 

, of cauterising the tips, 533. 

, of grease on the tips, 538. 

Vines, Mr., on cell-growth, 2. 

Vries, De, on turgescence, 2; on 
epinasty and hyponasty, 5, 269, 
270 ; the protection of hypocotyls 



during winter, 562 ; stolons aphe- 
liotropic, 109 ; the nyctitropic 
movement of leaves, 286; the po- 
sition of leaves influenced by epi- 
nasty, their own weight and apo- 
geotropism, 445 ; apogeotropism 
in petioles and midribs, 448 ; the 
stolons of strawberries. 459; the 
joints or pulvini of the Graminese, 
507. 

W. 

Watering, efl'ect of, on Porlieria 
hygrometrica^ 339-341. 

Wells, ' Essay on Dew,' 287, n. 

Wiesner, Prof., on the circumnuta- 
tion of the hypocotyl, 99, 100; on 
tlie hooked tip of climbing stems, 
274; observations on the efl'ect of 
bright sunshine on chlorophyll in 
leaves, 452 ; the eflects of an inter- 
mittent light, 462 ; on aerial roots, 
492; on special adaptations, 495. 

Wigandia, movement of leaves, 249. 

Williamson, Prof., on leaves of Dro- 
sera Capensis, 420. 

Wilson, Mr. A. S., on the movements 
of Swedish turnip leaves, 230, 301. 

Winkler on the protection of seed- 
lings, 109. 

Wistaria Sinensis, leaflets depressed 
at night, 359. 

, circumnutation with lateral 

light, 457. 

Z. 

Zea Mays^ circumnutation of cotyle- 
don, 65. 

Zea Mays, geotropic movement of 
radicles, 66. 

, sensitiveness of apex of radi- 
cle to contact, 177-179. 

, secondary radicles, 179. 

, heliotropic movements of seed- 
ling, 65, 426. 

, tips of radicles cauterised, 

592. 

Zukal, on the movements of Spiru- 
lina, 261, n. 



THE END. 




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LIBRARY OF CONGRESS 



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